The datetime module supplies classes for manipulating dates and times in both simple and complex ways. While date and time arithmetic is supported, the focus of the implementation is on efficient member extraction for output formatting and manipulation. For related functionality, see also the time and calendar modules.
There are two kinds of date and time objects: “naive” and “aware”. This distinction refers to whether the object has any notion of time zone, daylight saving time, or other kind of algorithmic or political time adjustment. Whether a naive datetime object represents Coordinated Universal Time (UTC), local time, or time in some other timezone is purely up to the program, just like it’s up to the program whether a particular number represents metres, miles, or mass. Naive datetime objects are easy to understand and to work with, at the cost of ignoring some aspects of reality.
For applications requiring more, datetime and time objects have an optional time zone information member, tzinfo, that can contain an instance of a subclass of the abstract tzinfo class. These tzinfo objects capture information about the offset from UTC time, the time zone name, and whether Daylight Saving Time is in effect. Note that no concrete tzinfo classes are supplied by the datetime module. Supporting timezones at whatever level of detail is required is up to the application. The rules for time adjustment across the world are more political than rational, and there is no standard suitable for every application.
The datetime module exports the following constants:
See also
Objects of these types are immutable.
Objects of the date type are always naive.
An object d of type time or datetime may be naive or aware. d is aware if d.tzinfo is not None and d.tzinfo.utcoffset(d) does not return None. If d.tzinfo is None, or if d.tzinfo is not None but d.tzinfo.utcoffset(d) returns None, d is naive.
The distinction between naive and aware doesn’t apply to timedelta objects.
Subclass relationships:
object
timedelta
tzinfo
time
date
datetime
A timedelta object represents a duration, the difference between two dates or times.
All arguments are optional and default to 0. Arguments may be integers or floats, and may be positive or negative.
Only days, seconds and microseconds are stored internally. Arguments are converted to those units:
and days, seconds and microseconds are then normalized so that the representation is unique, with
If any argument is a float and there are fractional microseconds, the fractional microseconds left over from all arguments are combined and their sum is rounded to the nearest microsecond. If no argument is a float, the conversion and normalization processes are exact (no information is lost).
If the normalized value of days lies outside the indicated range, OverflowError is raised.
Note that normalization of negative values may be surprising at first. For example,
>>> from datetime import timedelta
>>> d = timedelta(microseconds=-1)
>>> (d.days, d.seconds, d.microseconds)
(-1, 86399, 999999)
Class attributes are:
Note that, because of normalization, timedelta.max > -timedelta.min. -timedelta.max is not representable as a timedelta object.
Instance attributes (read-only):
| Attribute | Value |
|---|---|
| days | Between -999999999 and 999999999 inclusive |
| seconds | Between 0 and 86399 inclusive |
| microseconds | Between 0 and 999999 inclusive |
Supported operations:
| Operation | Result |
|---|---|
| t1 = t2 + t3 | Sum of t2 and t3. Afterwards t1-t2 == t3 and t1-t3 == t2 are true. (1) |
| t1 = t2 - t3 | Difference of t2 and t3. Afterwards t1 == t2 - t3 and t2 == t1 + t3 are true. (1) |
| t1 = t2 * i or t1 = i * t2 | Delta multiplied by an integer. Afterwards t1 // i == t2 is true, provided i != 0. |
| In general, t1 * i == t1 * (i-1) + t1 is true. (1) | |
| t1 = t2 // i | The floor is computed and the remainder (if any) is thrown away. (3) |
| +t1 | Returns a timedelta object with the same value. (2) |
| -t1 | equivalent to timedelta(-t1.days, -t1.seconds, -t1.microseconds), and to t1* -1. (1)(4) |
| abs(t) | equivalent to +t when t.days >= 0, and to -t when t.days < 0. (2) |
| str(t) | Returns a string in the form [D day[s], ][H]H:MM:SS[.UUUUUU], where D is negative for negative t. (5) |
| repr(t) | Returns a string in the form datetime.timedelta(D[, S[, U]]), where D is negative for negative t. (5) |
Notes:
This is exact, but may overflow.
This is exact, and cannot overflow.
Division by 0 raises ZeroDivisionError.
-timedelta.max is not representable as a timedelta object.
String representations of timedelta objects are normalized similarly to their internal representation. This leads to somewhat unusual results for negative timedeltas. For example:
>>> timedelta(hours=-5)
datetime.timedelta(-1, 68400)
>>> print(_)
-1 day, 19:00:00
In addition to the operations listed above timedelta objects support certain additions and subtractions with date and datetime objects (see below).
Comparisons of timedelta objects are supported with the timedelta object representing the smaller duration considered to be the smaller timedelta. In order to stop mixed-type comparisons from falling back to the default comparison by object address, when a timedelta object is compared to an object of a different type, TypeError is raised unless the comparison is == or !=. The latter cases return False or True, respectively.
timedelta objects are hashable (usable as dictionary keys), support efficient pickling, and in Boolean contexts, a timedelta object is considered to be true if and only if it isn’t equal to timedelta(0).
Example usage:
>>> from datetime import timedelta
>>> year = timedelta(days=365)
>>> another_year = timedelta(weeks=40, days=84, hours=23,
... minutes=50, seconds=600) # adds up to 365 days
>>> year == another_year
True
>>> ten_years = 10 * year
>>> ten_years, ten_years.days // 365
(datetime.timedelta(3650), 10)
>>> nine_years = ten_years - year
>>> nine_years, nine_years.days // 365
(datetime.timedelta(3285), 9)
>>> three_years = nine_years // 3;
>>> three_years, three_years.days // 365
(datetime.timedelta(1095), 3)
>>> abs(three_years - ten_years) == 2 * three_years + year
True
A date object represents a date (year, month and day) in an idealized calendar, the current Gregorian calendar indefinitely extended in both directions. January 1 of year 1 is called day number 1, January 2 of year 1 is called day number 2, and so on. This matches the definition of the “proleptic Gregorian” calendar in Dershowitz and Reingold’s book Calendrical Calculations, where it’s the base calendar for all computations. See the book for algorithms for converting between proleptic Gregorian ordinals and many other calendar systems.
All arguments are required. Arguments may be integers, in the following ranges:
If an argument outside those ranges is given, ValueError is raised.
Other constructors, all class methods:
Class attributes:
Instance attributes (read-only):
Supported operations:
| Operation | Result |
|---|---|
| date2 = date1 + timedelta | date2 is timedelta.days days removed from date1. (1) |
| date2 = date1 - timedelta | Computes date2 such that date2 + timedelta == date1. (2) |
| timedelta = date1 - date2 | (3) |
| date1 < date2 | date1 is considered less than date2 when date1 precedes date2 in time. (4) |
Notes:
Dates can be used as dictionary keys. In Boolean contexts, all date objects are considered to be true.
Instance methods:
Return a 3-tuple, (ISO year, ISO week number, ISO weekday).
The ISO calendar is a widely used variant of the Gregorian calendar. See http://www.phys.uu.nl/~vgent/calendar/isocalendar.htm for a good explanation.
The ISO year consists of 52 or 53 full weeks, and where a week starts on a Monday and ends on a Sunday. The first week of an ISO year is the first (Gregorian) calendar week of a year containing a Thursday. This is called week number 1, and the ISO year of that Thursday is the same as its Gregorian year.
For example, 2004 begins on a Thursday, so the first week of ISO year 2004 begins on Monday, 29 Dec 2003 and ends on Sunday, 4 Jan 2004, so that date(2003, 12, 29).isocalendar() == (2004, 1, 1) and date(2004, 1, 4).isocalendar() == (2004, 1, 7).
Example of counting days to an event:
>>> import time
>>> from datetime import date
>>> today = date.today()
>>> today
datetime.date(2007, 12, 5)
>>> today == date.fromtimestamp(time.time())
True
>>> my_birthday = date(today.year, 6, 24)
>>> if my_birthday < today:
... my_birthday = my_birthday.replace(year=today.year + 1)
>>> my_birthday
datetime.date(2008, 6, 24)
>>> time_to_birthday = abs(my_birthday - today)
>>> time_to_birthday.days
202
Example of working with date:
>>> from datetime import date
>>> d = date.fromordinal(730920) # 730920th day after 1. 1. 0001
>>> d
datetime.date(2002, 3, 11)
>>> t = d.timetuple()
>>> for i in t:
... print(i)
2002 # year
3 # month
11 # day
0
0
0
0 # weekday (0 = Monday)
70 # 70th day in the year
-1
>>> ic = d.isocalendar()
>>> for i in ic:
... print(i)
2002 # ISO year
11 # ISO week number
1 # ISO day number ( 1 = Monday )
>>> d.isoformat()
'2002-03-11'
>>> d.strftime("%d/%m/%y")
'11/03/02'
>>> d.strftime("%A %d. %B %Y")
'Monday 11. March 2002'
A datetime object is a single object containing all the information from a date object and a time object. Like a date object, datetime assumes the current Gregorian calendar extended in both directions; like a time object, datetime assumes there are exactly 3600*24 seconds in every day.
Constructor:
The year, month and day arguments are required. tzinfo may be None, or an instance of a tzinfo subclass. The remaining arguments may be integers, in the following ranges:
If an argument outside those ranges is given, ValueError is raised.
Other constructors, all class methods:
Return the current local date and time. If optional argument tz is None or not specified, this is like today(), but, if possible, supplies more precision than can be gotten from going through a time.time() timestamp (for example, this may be possible on platforms supplying the C gettimeofday() function).
Else tz must be an instance of a class tzinfo subclass, and the current date and time are converted to tz‘s time zone. In this case the result is equivalent to tz.fromutc(datetime.utcnow().replace(tzinfo=tz)). See also today(), utcnow().
Return the local date and time corresponding to the POSIX timestamp, such as is returned by time.time(). If optional argument tz is None or not specified, the timestamp is converted to the platform’s local date and time, and the returned datetime object is naive.
Else tz must be an instance of a class tzinfo subclass, and the timestamp is converted to tz‘s time zone. In this case the result is equivalent to tz.fromutc(datetime.utcfromtimestamp(timestamp).replace(tzinfo=tz)).
fromtimestamp() may raise ValueError, if the timestamp is out of the range of values supported by the platform C localtime() or gmtime() functions. It’s common for this to be restricted to years in 1970 through 2038. Note that on non-POSIX systems that include leap seconds in their notion of a timestamp, leap seconds are ignored by fromtimestamp(), and then it’s possible to have two timestamps differing by a second that yield identical datetime objects. See also utcfromtimestamp().
Class attributes:
Instance attributes (read-only):
Supported operations:
| Operation | Result |
|---|---|
| datetime2 = datetime1 + timedelta | (1) |
| datetime2 = datetime1 - timedelta | (2) |
| timedelta = datetime1 - datetime2 | (3) |
| datetime1 < datetime2 | Compares datetime to datetime. (4) |
datetime2 is a duration of timedelta removed from datetime1, moving forward in time if timedelta.days > 0, or backward if timedelta.days < 0. The result has the same tzinfo member as the input datetime, and datetime2 - datetime1 == timedelta after. OverflowError is raised if datetime2.year would be smaller than MINYEAR or larger than MAXYEAR. Note that no time zone adjustments are done even if the input is an aware object.
Computes the datetime2 such that datetime2 + timedelta == datetime1. As for addition, the result has the same tzinfo member as the input datetime, and no time zone adjustments are done even if the input is aware. This isn’t quite equivalent to datetime1 + (-timedelta), because -timedelta in isolation can overflow in cases where datetime1 - timedelta does not.
Subtraction of a datetime from a datetime is defined only if both operands are naive, or if both are aware. If one is aware and the other is naive, TypeError is raised.
If both are naive, or both are aware and have the same tzinfo member, the tzinfo members are ignored, and the result is a timedelta object t such that datetime2 + t == datetime1. No time zone adjustments are done in this case.
If both are aware and have different tzinfo members, a-b acts as if a and b were first converted to naive UTC datetimes first. The result is (a.replace(tzinfo=None) - a.utcoffset()) - (b.replace(tzinfo=None) - b.utcoffset()) except that the implementation never overflows.
datetime1 is considered less than datetime2 when datetime1 precedes datetime2 in time.
If one comparand is naive and the other is aware, TypeError is raised. If both comparands are aware, and have the same tzinfo member, the common tzinfo member is ignored and the base datetimes are compared. If both comparands are aware and have different tzinfo members, the comparands are first adjusted by subtracting their UTC offsets (obtained from self.utcoffset()).
Note
In order to stop comparison from falling back to the default scheme of comparing object addresses, datetime comparison normally raises TypeError if the other comparand isn’t also a datetime object. However, NotImplemented is returned instead if the other comparand has a timetuple() attribute. This hook gives other kinds of date objects a chance at implementing mixed-type comparison. If not, when a datetime object is compared to an object of a different type, TypeError is raised unless the comparison is == or !=. The latter cases return False or True, respectively.
datetime objects can be used as dictionary keys. In Boolean contexts, all datetime objects are considered to be true.
Instance methods:
Return a datetime object with new tzinfo member tz, adjusting the date and time members so the result is the same UTC time as self, but in tz‘s local time.
tz must be an instance of a tzinfo subclass, and its utcoffset() and dst() methods must not return None. self must be aware (self.tzinfo must not be None, and self.utcoffset() must not return None).
If self.tzinfo is tz, self.astimezone(tz) is equal to self: no adjustment of date or time members is performed. Else the result is local time in time zone tz, representing the same UTC time as self: after astz = dt.astimezone(tz), astz - astz.utcoffset() will usually have the same date and time members as dt - dt.utcoffset(). The discussion of class tzinfo explains the cases at Daylight Saving Time transition boundaries where this cannot be achieved (an issue only if tz models both standard and daylight time).
If you merely want to attach a time zone object tz to a datetime dt without adjustment of date and time members, use dt.replace(tzinfo=tz). If you merely want to remove the time zone object from an aware datetime dt without conversion of date and time members, use dt.replace(tzinfo=None).
Note that the default tzinfo.fromutc() method can be overridden in a tzinfo subclass to affect the result returned by astimezone(). Ignoring error cases, astimezone() acts like:
def astimezone(self, tz):
if self.tzinfo is tz:
return self
# Convert self to UTC, and attach the new time zone object.
utc = (self - self.utcoffset()).replace(tzinfo=tz)
# Convert from UTC to tz's local time.
return tz.fromutc(utc)
If datetime instance d is naive, this is the same as d.timetuple() except that tm_isdst is forced to 0 regardless of what d.dst() returns. DST is never in effect for a UTC time.
If d is aware, d is normalized to UTC time, by subtracting d.utcoffset(), and a time.struct_time for the normalized time is returned. tm_isdst is forced to 0. Note that the result’s tm_year member may be MINYEAR-1 or MAXYEAR+1, if d.year was MINYEAR or MAXYEAR and UTC adjustment spills over a year boundary.
Return a string representing the date and time in ISO 8601 format, YYYY-MM-DDTHH:MM:SS.mmmmmm or, if microsecond is 0, YYYY-MM-DDTHH:MM:SS
If utcoffset() does not return None, a 6-character string is appended, giving the UTC offset in (signed) hours and minutes: YYYY-MM-DDTHH:MM:SS.mmmmmm+HH:MM or, if microsecond is 0 YYYY-MM-DDTHH:MM:SS+HH:MM
The optional argument sep (default 'T') is a one-character separator, placed between the date and time portions of the result. For example,
>>> from datetime import tzinfo, timedelta, datetime
>>> class TZ(tzinfo):
... def utcoffset(self, dt): return timedelta(minutes=-399)
...
>>> datetime(2002, 12, 25, tzinfo=TZ()).isoformat(' ')
'2002-12-25 00:00:00-06:39'
Examples of working with datetime objects:
>>> from datetime import datetime, date, time
>>> # Using datetime.combine()
>>> d = date(2005, 7, 14)
>>> t = time(12, 30)
>>> datetime.combine(d, t)
datetime.datetime(2005, 7, 14, 12, 30)
>>> # Using datetime.now() or datetime.utcnow()
>>> datetime.now()
datetime.datetime(2007, 12, 6, 16, 29, 43, 79043) # GMT +1
>>> datetime.utcnow()
datetime.datetime(2007, 12, 6, 15, 29, 43, 79060)
>>> # Using datetime.strptime()
>>> dt = datetime.strptime("21/11/06 16:30", "%d/%m/%y %H:%M")
>>> dt
datetime.datetime(2006, 11, 21, 16, 30)
>>> # Using datetime.timetuple() to get tuple of all attributes
>>> tt = dt.timetuple()
>>> for it in tt:
... print(it)
...
2006 # year
11 # month
21 # day
16 # hour
30 # minute
0 # second
1 # weekday (0 = Monday)
325 # number of days since 1st January
-1 # dst - method tzinfo.dst() returned None
>>> # Date in ISO format
>>> ic = dt.isocalendar()
>>> for it in ic:
... print(it)
...
2006 # ISO year
47 # ISO week
2 # ISO weekday
>>> # Formatting datetime
>>> dt.strftime("%A, %d. %B %Y %I:%M%p")
'Tuesday, 21. November 2006 04:30PM'
Using datetime with tzinfo:
>>> from datetime import timedelta, datetime, tzinfo
>>> class GMT1(tzinfo):
... def __init__(self): # DST starts last Sunday in March
... d = datetime(dt.year, 4, 1) # ends last Sunday in October
... self.dston = d - timedelta(days=d.weekday() + 1)
... d = datetime(dt.year, 11, 1)
... self.dstoff = d - timedelta(days=d.weekday() + 1)
... def utcoffset(self, dt):
... return timedelta(hours=1) + self.dst(dt)
... def dst(self, dt):
... if self.dston <= dt.replace(tzinfo=None) < self.dstoff:
... return timedelta(hours=1)
... else:
... return timedelta(0)
... def tzname(self,dt):
... return "GMT +1"
...
>>> class GMT2(tzinfo):
... def __init__(self):
... d = datetime(dt.year, 4, 1)
... self.dston = d - timedelta(days=d.weekday() + 1)
... d = datetime(dt.year, 11, 1)
... self.dstoff = d - timedelta(days=d.weekday() + 1)
... def utcoffset(self, dt):
... return timedelta(hours=1) + self.dst(dt)
... def dst(self, dt):
... if self.dston <= dt.replace(tzinfo=None) < self.dstoff:
... return timedelta(hours=2)
... else:
... return timedelta(0)
... def tzname(self,dt):
... return "GMT +2"
...
>>> gmt1 = GMT1()
>>> # Daylight Saving Time
>>> dt1 = datetime(2006, 11, 21, 16, 30, tzinfo=gmt1)
>>> dt1.dst()
datetime.timedelta(0)
>>> dt1.utcoffset()
datetime.timedelta(0, 3600)
>>> dt2 = datetime(2006, 6, 14, 13, 0, tzinfo=gmt1)
>>> dt2.dst()
datetime.timedelta(0, 3600)
>>> dt2.utcoffset()
datetime.timedelta(0, 7200)
>>> # Convert datetime to another time zone
>>> dt3 = dt2.astimezone(GMT2())
>>> dt3 # doctest: +ELLIPSIS
datetime.datetime(2006, 6, 14, 14, 0, tzinfo=<GMT2 object at 0x...>)
>>> dt2 # doctest: +ELLIPSIS
datetime.datetime(2006, 6, 14, 13, 0, tzinfo=<GMT1 object at 0x...>)
>>> dt2.utctimetuple() == dt3.utctimetuple()
True
A time object represents a (local) time of day, independent of any particular day, and subject to adjustment via a tzinfo object.
All arguments are optional. tzinfo may be None, or an instance of a tzinfo subclass. The remaining arguments may be integers, in the following ranges:
If an argument outside those ranges is given, ValueError is raised. All default to 0 except tzinfo, which defaults to None.
Class attributes:
Instance attributes (read-only):
Supported operations:
Instance methods:
Example:
>>> from datetime import time, tzinfo
>>> class GMT1(tzinfo):
... def utcoffset(self, dt):
... return timedelta(hours=1)
... def dst(self, dt):
... return timedelta(0)
... def tzname(self,dt):
... return "Europe/Prague"
...
>>> t = time(12, 10, 30, tzinfo=GMT1())
>>> t # doctest: +ELLIPSIS
datetime.time(12, 10, 30, tzinfo=<GMT1 object at 0x...>)
>>> gmt = GMT1()
>>> t.isoformat()
'12:10:30+01:00'
>>> t.dst()
datetime.timedelta(0)
>>> t.tzname()
'Europe/Prague'
>>> t.strftime("%H:%M:%S %Z")
'12:10:30 Europe/Prague'
tzinfo is an abstract base class, meaning that this class should not be instantiated directly. You need to derive a concrete subclass, and (at least) supply implementations of the standard tzinfo methods needed by the datetime methods you use. The datetime module does not supply any concrete subclasses of tzinfo.
An instance of (a concrete subclass of) tzinfo can be passed to the constructors for datetime and time objects. The latter objects view their members as being in local time, and the tzinfo object supports methods revealing offset of local time from UTC, the name of the time zone, and DST offset, all relative to a date or time object passed to them.
Special requirement for pickling: A tzinfo subclass must have an __init__() method that can be called with no arguments, else it can be pickled but possibly not unpickled again. This is a technical requirement that may be relaxed in the future.
A concrete subclass of tzinfo may need to implement the following methods. Exactly which methods are needed depends on the uses made of aware datetime objects. If in doubt, simply implement all of them.
Return offset of local time from UTC, in minutes east of UTC. If local time is west of UTC, this should be negative. Note that this is intended to be the total offset from UTC; for example, if a tzinfo object represents both time zone and DST adjustments, utcoffset() should return their sum. If the UTC offset isn’t known, return None. Else the value returned must be a timedelta object specifying a whole number of minutes in the range -1439 to 1439 inclusive (1440 = 24*60; the magnitude of the offset must be less than one day). Most implementations of utcoffset() will probably look like one of these two:
return CONSTANT # fixed-offset class
return CONSTANT + self.dst(dt) # daylight-aware class
If utcoffset() does not return None, dst() should not return None either.
The default implementation of utcoffset() raises NotImplementedError.
Return the daylight saving time (DST) adjustment, in minutes east of UTC, or None if DST information isn’t known. Return timedelta(0) if DST is not in effect. If DST is in effect, return the offset as a timedelta object (see utcoffset() for details). Note that DST offset, if applicable, has already been added to the UTC offset returned by utcoffset(), so there’s no need to consult dst() unless you’re interested in obtaining DST info separately. For example, datetime.timetuple() calls its tzinfo member’s dst() method to determine how the tm_isdst flag should be set, and tzinfo.fromutc() calls dst() to account for DST changes when crossing time zones.
An instance tz of a tzinfo subclass that models both standard and daylight times must be consistent in this sense:
tz.utcoffset(dt) - tz.dst(dt)
must return the same result for every datetime dt with dt.tzinfo == tz For sane tzinfo subclasses, this expression yields the time zone’s “standard offset”, which should not depend on the date or the time, but only on geographic location. The implementation of datetime.astimezone() relies on this, but cannot detect violations; it’s the programmer’s responsibility to ensure it. If a tzinfo subclass cannot guarantee this, it may be able to override the default implementation of tzinfo.fromutc() to work correctly with astimezone() regardless.
Most implementations of dst() will probably look like one of these two:
def dst(self):
# a fixed-offset class: doesn't account for DST
return timedelta(0)
or
def dst(self):
# Code to set dston and dstoff to the time zone's DST
# transition times based on the input dt.year, and expressed
# in standard local time. Then
if dston <= dt.replace(tzinfo=None) < dstoff:
return timedelta(hours=1)
else:
return timedelta(0)
The default implementation of dst() raises NotImplementedError.
Return the time zone name corresponding to the datetime object dt, as a string. Nothing about string names is defined by the datetime module, and there’s no requirement that it mean anything in particular. For example, “GMT”, “UTC”, “-500”, “-5:00”, “EDT”, “US/Eastern”, “America/New York” are all valid replies. Return None if a string name isn’t known. Note that this is a method rather than a fixed string primarily because some tzinfo subclasses will wish to return different names depending on the specific value of dt passed, especially if the tzinfo class is accounting for daylight time.
The default implementation of tzname() raises NotImplementedError.
These methods are called by a datetime or time object, in response to their methods of the same names. A datetime object passes itself as the argument, and a time object passes None as the argument. A tzinfo subclass’s methods should therefore be prepared to accept a dt argument of None, or of class datetime.
When None is passed, it’s up to the class designer to decide the best response. For example, returning None is appropriate if the class wishes to say that time objects don’t participate in the tzinfo protocols. It may be more useful for utcoffset(None) to return the standard UTC offset, as there is no other convention for discovering the standard offset.
When a datetime object is passed in response to a datetime method, dt.tzinfo is the same object as self. tzinfo methods can rely on this, unless user code calls tzinfo methods directly. The intent is that the tzinfo methods interpret dt as being in local time, and not need worry about objects in other timezones.
There is one more tzinfo method that a subclass may wish to override:
This is called from the default datetime.astimezone() implementation. When called from that, dt.tzinfo is self, and dt‘s date and time members are to be viewed as expressing a UTC time. The purpose of fromutc() is to adjust the date and time members, returning an equivalent datetime in self‘s local time.
Most tzinfo subclasses should be able to inherit the default fromutc() implementation without problems. It’s strong enough to handle fixed-offset time zones, and time zones accounting for both standard and daylight time, and the latter even if the DST transition times differ in different years. An example of a time zone the default fromutc() implementation may not handle correctly in all cases is one where the standard offset (from UTC) depends on the specific date and time passed, which can happen for political reasons. The default implementations of astimezone() and fromutc() may not produce the result you want if the result is one of the hours straddling the moment the standard offset changes.
Skipping code for error cases, the default fromutc() implementation acts like:
def fromutc(self, dt):
# raise ValueError error if dt.tzinfo is not self
dtoff = dt.utcoffset()
dtdst = dt.dst()
# raise ValueError if dtoff is None or dtdst is None
delta = dtoff - dtdst # this is self's standard offset
if delta:
dt += delta # convert to standard local time
dtdst = dt.dst()
# raise ValueError if dtdst is None
if dtdst:
return dt + dtdst
else:
return dt
Example tzinfo classes:
from datetime import tzinfo, timedelta, datetime
ZERO = timedelta(0)
HOUR = timedelta(hours=1)
# A UTC class.
class UTC(tzinfo):
"""UTC"""
def utcoffset(self, dt):
return ZERO
def tzname(self, dt):
return "UTC"
def dst(self, dt):
return ZERO
utc = UTC()
# A class building tzinfo objects for fixed-offset time zones.
# Note that FixedOffset(0, "UTC") is a different way to build a
# UTC tzinfo object.
class FixedOffset(tzinfo):
"""Fixed offset in minutes east from UTC."""
def __init__(self, offset, name):
self.__offset = timedelta(minutes = offset)
self.__name = name
def utcoffset(self, dt):
return self.__offset
def tzname(self, dt):
return self.__name
def dst(self, dt):
return ZERO
# A class capturing the platform's idea of local time.
import time as _time
STDOFFSET = timedelta(seconds = -_time.timezone)
if _time.daylight:
DSTOFFSET = timedelta(seconds = -_time.altzone)
else:
DSTOFFSET = STDOFFSET
DSTDIFF = DSTOFFSET - STDOFFSET
class LocalTimezone(tzinfo):
def utcoffset(self, dt):
if self._isdst(dt):
return DSTOFFSET
else:
return STDOFFSET
def dst(self, dt):
if self._isdst(dt):
return DSTDIFF
else:
return ZERO
def tzname(self, dt):
return _time.tzname[self._isdst(dt)]
def _isdst(self, dt):
tt = (dt.year, dt.month, dt.day,
dt.hour, dt.minute, dt.second,
dt.weekday(), 0, -1)
stamp = _time.mktime(tt)
tt = _time.localtime(stamp)
return tt.tm_isdst > 0
Local = LocalTimezone()
# A complete implementation of current DST rules for major US time zones.
def first_sunday_on_or_after(dt):
days_to_go = 6 - dt.weekday()
if days_to_go:
dt += timedelta(days_to_go)
return dt
# US DST Rules
#
# This is a simplified (i.e., wrong for a few cases) set of rules for US
# DST start and end times. For a complete and up-to-date set of DST rules
# and timezone definitions, visit the Olson Database (or try pytz):
# http://www.twinsun.com/tz/tz-link.htm
# http://sourceforge.net/projects/pytz/ (might not be up-to-date)
#
# In the US, since 2007, DST starts at 2am (standard time) on the second
# Sunday in March, which is the first Sunday on or after Mar 8.
DSTSTART_2007 = datetime(1, 3, 8, 2)
# and ends at 2am (DST time; 1am standard time) on the first Sunday of Nov.
DSTEND_2007 = datetime(1, 11, 1, 1)
# From 1987 to 2006, DST used to start at 2am (standard time) on the first
# Sunday in April and to end at 2am (DST time; 1am standard time) on the last
# Sunday of October, which is the first Sunday on or after Oct 25.
DSTSTART_1987_2006 = datetime(1, 4, 1, 2)
DSTEND_1987_2006 = datetime(1, 10, 25, 1)
# From 1967 to 1986, DST used to start at 2am (standard time) on the last
# Sunday in April (the one on or after April 24) and to end at 2am (DST time;
# 1am standard time) on the last Sunday of October, which is the first Sunday
# on or after Oct 25.
DSTSTART_1967_1986 = datetime(1, 4, 24, 2)
DSTEND_1967_1986 = DSTEND_1987_2006
class USTimeZone(tzinfo):
def __init__(self, hours, reprname, stdname, dstname):
self.stdoffset = timedelta(hours=hours)
self.reprname = reprname
self.stdname = stdname
self.dstname = dstname
def __repr__(self):
return self.reprname
def tzname(self, dt):
if self.dst(dt):
return self.dstname
else:
return self.stdname
def utcoffset(self, dt):
return self.stdoffset + self.dst(dt)
def dst(self, dt):
if dt is None or dt.tzinfo is None:
# An exception may be sensible here, in one or both cases.
# It depends on how you want to treat them. The default
# fromutc() implementation (called by the default astimezone()
# implementation) passes a datetime with dt.tzinfo is self.
return ZERO
assert dt.tzinfo is self
# Find start and end times for US DST. For years before 1967, return
# ZERO for no DST.
if 2006 < dt.year:
dststart, dstend = DSTSTART_2007, DSTEND_2007
elif 1986 < dt.year < 2007:
dststart, dstend = DSTSTART_1987_2006, DSTEND_1987_2006
elif 1966 < dt.year < 1987:
dststart, dstend = DSTSTART_1967_1986, DSTEND_1967_1986
else:
return ZERO
start = first_sunday_on_or_after(dststart.replace(year=dt.year))
end = first_sunday_on_or_after(dstend.replace(year=dt.year))
# Can't compare naive to aware objects, so strip the timezone from
# dt first.
if start <= dt.replace(tzinfo=None) < end:
return HOUR
else:
return ZERO
Eastern = USTimeZone(-5, "Eastern", "EST", "EDT")
Central = USTimeZone(-6, "Central", "CST", "CDT")
Mountain = USTimeZone(-7, "Mountain", "MST", "MDT")
Pacific = USTimeZone(-8, "Pacific", "PST", "PDT")
Note that there are unavoidable subtleties twice per year in a tzinfo subclass accounting for both standard and daylight time, at the DST transition points. For concreteness, consider US Eastern (UTC -0500), where EDT begins the minute after 1:59 (EST) on the second Sunday in March, and ends the minute after 1:59 (EDT) on the first Sunday in November:
UTC 3:MM 4:MM 5:MM 6:MM 7:MM 8:MM
EST 22:MM 23:MM 0:MM 1:MM 2:MM 3:MM
EDT 23:MM 0:MM 1:MM 2:MM 3:MM 4:MM
start 22:MM 23:MM 0:MM 1:MM 3:MM 4:MM
end 23:MM 0:MM 1:MM 1:MM 2:MM 3:MM
When DST starts (the “start” line), the local wall clock leaps from 1:59 to 3:00. A wall time of the form 2:MM doesn’t really make sense on that day, so astimezone(Eastern) won’t deliver a result with hour == 2 on the day DST begins. In order for astimezone() to make this guarantee, the rzinfo.dst() method must consider times in the “missing hour” (2:MM for Eastern) to be in daylight time.
When DST ends (the “end” line), there’s a potentially worse problem: there’s an hour that can’t be spelled unambiguously in local wall time: the last hour of daylight time. In Eastern, that’s times of the form 5:MM UTC on the day daylight time ends. The local wall clock leaps from 1:59 (daylight time) back to 1:00 (standard time) again. Local times of the form 1:MM are ambiguous. astimezone() mimics the local clock’s behavior by mapping two adjacent UTC hours into the same local hour then. In the Eastern example, UTC times of the form 5:MM and 6:MM both map to 1:MM when converted to Eastern. In order for astimezone() to make this guarantee, the tzinfo.dst() method must consider times in the “repeated hour” to be in standard time. This is easily arranged, as in the example, by expressing DST switch times in the time zone’s standard local time.
Applications that can’t bear such ambiguities should avoid using hybrid tzinfo subclasses; there are no ambiguities when using UTC, or any other fixed-offset tzinfo subclass (such as a class representing only EST (fixed offset -5 hours), or only EDT (fixed offset -4 hours)).
date, datetime, and time objects all support a strftime(format) method, to create a string representing the time under the control of an explicit format string. Broadly speaking, d.strftime(fmt) acts like the time module’s time.strftime(fmt, d.timetuple()) although not all objects support a timetuple() method.
Conversely, the datetime.strptime() class method creates a datetime object from a string representing a date and time and a corresponding format string. datetime.strptime(date_string, format) is equivalent to datetime(*(time.strptime(date_string, format)[0:6])).
For time objects, the format codes for year, month, and day should not be used, as time objects have no such values. If they’re used anyway, 1900 is substituted for the year, and 1 for the month and day.
For date objects, the format codes for hours, minutes, seconds, and microseconds should not be used, as date objects have no such values. If they’re used anyway, 0 is substituted for them.
For a naive object, the %z and %Z format codes are replaced by empty strings.
For an aware object:
The full set of format codes supported varies across platforms, because Python calls the platform C library’s strftime() function, and platform variations are common.
The following is a list of all the format codes that the C standard (1989 version) requires, and these work on all platforms with a standard C implementation. Note that the 1999 version of the C standard added additional format codes.
The exact range of years for which strftime() works also varies across platforms. Regardless of platform, years before 1900 cannot be used.
| Directive | Meaning | Notes |
|---|---|---|
| %a | Locale’s abbreviated weekday name. | |
| %A | Locale’s full weekday name. | |
| %b | Locale’s abbreviated month name. | |
| %B | Locale’s full month name. | |
| %c | Locale’s appropriate date and time representation. | |
| %d | Day of the month as a decimal number [01,31]. | |
| %f | Microsecond as a decimal number [0,999999], zero-padded on the left | (1) |
| %H | Hour (24-hour clock) as a decimal number [00,23]. | |
| %I | Hour (12-hour clock) as a decimal number [01,12]. | |
| %j | Day of the year as a decimal number [001,366]. | |
| %m | Month as a decimal number [01,12]. | |
| %M | Minute as a decimal number [00,59]. | |
| %p | Locale’s equivalent of either AM or PM. | (2) |
| %S | Second as a decimal number [00,61]. | (3) |
| %U | Week number of the year (Sunday as the first day of the week) as a decimal number [00,53]. All days in a new year preceding the first Sunday are considered to be in week 0. | (4) |
| %w | Weekday as a decimal number [0(Sunday),6]. | |
| %W | Week number of the year (Monday as the first day of the week) as a decimal number [00,53]. All days in a new year preceding the first Monday are considered to be in week 0. | (4) |
| %x | Locale’s appropriate date representation. | |
| %X | Locale’s appropriate time representation. | |
| %y | Year without century as a decimal number [00,99]. | |
| %Y | Year with century as a decimal number. | |
| %z | UTC offset in the form +HHMM or -HHMM (empty string if the the object is naive). | (5) |
| %Z | Time zone name (empty string if the object is naive). | |
| %% | A literal '%' character. |
Notes: