On: Ask HN: Did you encounter any leap year bugs today...

[bregma]

Python.

    cls = <class 'datetime.datetime'>, data_string = 'Feb 29 04:55:03.687' format = '%b %d %H:%M:%S.%f'
    E       ValueError: day is out of range for month

[ptx → bregma]

As mentioned by sibling comments, it's because you're not specifying a year. If you change the day to the 28th you'll see that it defaults to the year 1900:

  >>> datetime.strptime('Feb 28 04:55:03.687', '%b %d %H:%M:%S.%f')
  datetime.datetime(1900, 2, 28, 4, 55, 3, 687000)

  >>> datetime.strptime('Feb 28 13:37:06.942', '%b %d %H:%M:%S.%f') 
  datetime.datetime(1900, 2, 28, 13, 37, 6, 942000)

[OJFord → ptx]

That makes it weird though, because 1900 was a leap year? I sort of get it, but it's a slightly odd and inconsistent decision.

Edit: no it's not, it's absolutely correct, leap years just aren't as simple as I thought!

[jesprenj → OJFord]

It wasn't. https://learn.microsoft.com/sl-SI/office/troubleshoot/excel/...

[OJFord → jesprenj]

Crikey - more helpful to me is the page linked from there - https://learn.microsoft.com/sl-SI/office/troubleshoot/excel/...

> However, there is still a small error that must be accounted for. To eliminate this error, the Gregorian calendar stipulates that a year that is evenly divisible by 100 (for example, 1900) is a leap year only if it is also evenly divisible by 400.

> For this reason, the following years are not leap years:

> 1700, 1800, 1900, 2100, 2200, 2300, 2500, 2600

I had no idea!

[ahazred8ta → OJFord]

"We establish that a bissextile [366th day] shall be inserted every four years (as with the present custom), except in centennial years. So the years 1700, 1800, and 1900 will not be leap years. Assuredly, the year 2000 will have an extra day in it." -- Greg XIII, 1582

[pxx → OJFord]

1900 was not a leap year... It's 0 mod 4, yes, but it's 0 mod 100 and not 0 mod 400

[OJFord → pxx]

Yep, thanks, I just wasn't aware of the latter rule at all. Time to re-read Falsehoods perhaps!

[derefr → bregma]

That doesn’t seem incorrect; given that no year is specified, it seems like it’s evaluating the constraint in the context of an implicit default year. (1970? 0CE?)

The confusing part, to me, is that Python would consider the above string to be parsed into a date in the first place, given that it has no year.

[herpderperator → bregma]

Interesting... I suppose that is because there is no year? What year does it default to? Can you show your exact line of code?

[nicholasjarnold → bregma]

confirmed. and interesting/unexpected! this breaks:

datetime.strptime('Feb 29 13:37:06.942', '%b %d %H:%M:%S.%f')

edit: added code example. import datetime from datetime first obvi

[LeoPanthera → bregma]

That's because you didn't specify a year.