[perl4ever]

What if you take March 31st and add one month to it? (I assume you can do that somehow)

[deathanatos]

Not with the standard library. (The "timedelta" class only expresses in units up to days, as it is ambiguous how long a month is.

There's a nice package called "python-dateutil" that includes a "relativedelta" class; adding a month to March 31 results in April 30:

  In [7]: datetime.datetime(2020, 3, 31) + dateutil.relativedelta.relativedelta(months=1)                                
  Out[7]: datetime.datetime(2020, 4, 30, 0, 0)

Adding a year to a leap day:

  In [8]: datetime.datetime(2020, 2, 29) + dateutil.relativedelta.relativedelta(years=1)                                 
  Out[8]: datetime.datetime(2021, 2, 28, 0, 0)

The exact duration that relativedelta adds depends on what you add it to. (Hence the name.) But the results tend to match up with human expectations.

[wruza]

You get April 30. Idk about all libraries, but date-fns and few others I worked with do it right.

Also, if you add one month to Apr 30 and two months to Mar 31, you’ll see that month addition is not commutative-y and one should operate on distances from a base date, not in an incremental way.