Adding 1 month to February 28th results in March 28th

[pganssle]

Migrated from launchpad issue #1292043.

Quoting Florian Müller on 2014-03-13

I actually don't know, whether this is a bug but it was not the behavior I expected so I report it.

Consider the following code:

>>> datetime.date(2014,12,31) + relativedelta(months=1) + relativedelta(months=1) + relativedelta(months=1)
datetime.date(2015, 3, 28)
>>> datetime.date(2014,12,31) + relativedelta(months=3)
datetime.date(2015, 3, 31)

I really expected both examples to result in the same date.

Or want may I do to achieve the same result (other than manually check for february 28th/29th)

[pganssle]

@jajadinimueter (assuming you are the same here and on LP - seems likely):

Thanks for reporting this. This is an order of operations issue, and is the expected behavior. The problem is that any addition between a datetime and a relativedelta has to return a valid date. So in your example:

>>> datetime.date(2014,12,31) + relativedelta(months=1) + relativedelta(months=1) + relativedelta(months=1)
datetime.date(2015, 3, 28)

This is the same as:

>>> (((datetime.date(2014,12,31) + relativedelta(months=1)) + relativedelta(months=1)) + relativedelta(months=1))
datetime.date(2015, 3, 28)

So the first operation resolves to datetime.date(2014, 1, 31), the second one resolves to datetime.date(2014, 2, 28) (because Feb 31 is not a valid date), and the third resolves to datetime.date(2014, 3, 28). I'm not sure what your specific use case is (e.g. why you want to use 3 relativedeltas instead of one), but in your example, you can fix it by doing all the relativedelta addition first:

>>> datetime.date(2014,12,31) + (relativedelta(months=1) + relativedelta(months=1) + relativedelta(months=1))
datetime.date(2015, 3, 31)