A Guide to Python Datetimes

2018-01-03

Technology

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After a harrowing journey of debugging datetime issues in Python, I've decided to solve this once-and-for-all (or at least give myself a resource for the next n times I have to deal with this). Here's what you (read: I) need to know.

There are three datetime formats to be aware of in Python:

  • struct_time (naive)
  • POSIX (aware)
  • datetime (naive or aware)

This can be further broken into six datetime states:

  • struct_time (local)
  • struct_time (UTC)
  • POSIX
  • Aware datetime (Python 2)
  • Aware datetime (Python 3)
  • Naive datetime (local)
  • Naive datetime (UTC)

To get the current time in any of the above formats, run:

To Get ThisDo This
struct_time (local)time.localtime()
struct_time (UTC)time.gmtime()
POSIXtime.time()
Aware datetime (Py2)datetime.datetime.now(pytz.timezone('UTC'))
Aware datetime (Py3)datetime.datetime.now(datetime.timezone.utc)
Naive datetime (local)datetime.datetime.now()
Naive datetime (UTC)datetime.datetime.utcnow()

Note that Python 2 support for timezones is a bit lacking, as you can see above. To bridge the gap, you'll likely need to install the tzlocal and pytz modules (python2 -m pip install pytz tzlocal).

To convert between datetime formats, use the following table. You can find the code for generating this table on my GitHub. Pull requests for any other direct conversion methods I've missed are gladly accepted -- note, though, that the below table attempts to avoid redundancy; sometimes you'll need to convert through intermediate formats:

Row -> ColPOSIXstruct_time (local)struct_time (UTC)Naive datetime (local)Naive datetime (UTC)Aware datetime (Py2)Aware datetime (Py3)
POSIX-time.localtime(x)time.gmtime(x)datetime.datetime.fromtimestamp(x)datetime.datetime.utcfromtimestamp(x)datetime.datetime.fromtimestamp(x, pytz.timezone('UTC'))datetime.datetime.fromtimestamp(x, datetime.timezone.utc)
struct_time (local)time.mktime(x)--datetime.datetime(*x[:6])---
struct_time (UTC)calendar.timegm(x)---datetime.datetime(*x[:6])datetime.datetime(*x[:6], tzinfo=pytz.timezone('UTC'))datetime.datetime(*x[:6], tzinfo=datetime.timezone.utc)
Naive datetime (local)-----x.replace(tzinfo=tzlocal.get_localzone())x.astimezone()
Naive datetime (UTC)calendar.timegm(x.utctimetuple())-x.utctimetuple()--x.replace(tzinfo=pytz.timezone('UTC'))x.replace(tzinfo=datetime.timezone.utc)
Aware datetime (Py2)calendar.timegm(x.utctimetuple())-x.utctimetuple()x.astimezone(tzlocal.get_localzone()).replace(tzinfo=None)x.astimezone(pytz.timezone('UTC')).replace(tzinfo=None)--
Aware datetime (Py3)calendar.timegm(x.utctimetuple())-x.utctimetuple()x.astimezone().replace(tzinfo=None)x.astimezone(datetime.timezone.utc).replace(tzinfo=None)--

I've used UTC as the standard timezone when converting to / creating aware datetime entities, but you can use any other timezone you'd like. To convert between timezones with aware datetime entities, you can do:

  • Python 2: x.astimezone(pytz.timezone('UTC')); x.astimezone(pytz.timezone('US/Pacific'))
  • Python 3: x.astimezone(datetime.timezone.utc); x.astimezone(datetime.timezone(datetime.timedelta(hours=-8)))