Some of our tests were constructing expected dates, one year from today, with the following code:

from datetime import datetime

now = datetime.utcnow()
then = datetime(now.year + 1, now.month, now.day)

Of course if you run this code today, then it tries to construct a datetime for February 29th 2013, which fails because that date doesn't exist.

When I posted this on twitter a few people suggested that instead we should have used timedelta(days=365) instead. Again, this works most of the time - but if you want a date exactly one year from now it will fail in leap years when used before February 29th:

>>> from datetime import datetime, timedelta
>>> datetime(2012, 2, 27) + timedelta(days=365)
datetime.datetime(2013, 2, 26, 0, 0)

The correct fix is to use the wonderful dateutil module, in particular the dateutil.relativedelta.relativedelta:

>>> from datetime import datetime
>>> from dateutil.relativedelta import relativedelta
>>> datetime.utcnow() + relativedelta(years=1)
datetime.datetime(2013, 2, 28, 15, 20, 21, 546755)
>>> datetime(2012, 2, 27) + relativedelta(years=1)
datetime.datetime(2013, 2, 27, 0, 0)

And as another hint, always use datetime.utcnow() instead of datetime.now() to avoid horrible timezone nightmares (exactly which timezone are your servers in?).


Source: http://www.voidspace.org.uk/python/weblog/arch_d7_2012_02_25.shtml