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It's not (only?) related to leap years. It appears to be related to the month of April and some other weird stuff. -- leap year 2012: 2019-01-31 to 201

It's not (only?) related to leap years. It appears to be related to the month of April and some other weird stuff. -- leap year 2012: 2019-01-31 to 2012-01-30 --> 7 years 1 day 2012-01-30 to 2019-01-31 --> 7 years 1 day 2019-01-31 to 2012-02-29 --> 6 years 11 months <---- Weird stuff 2012-02-29 to 2019-01-31 --> 6 years 11 months 3 days <---- Weird stuff 2019-01-31 to 2012-02-30 --> 6 years 10 months 30 days (2012-02-30 does not exist) 2012-02-30 to 2019-01-31 --> 6 years 10 months 30 days (2012-02-30 does not exist) 2019-01-31 to 2012-03-30 --> 6 years 10 months 1 day 2012-03-30 to 2019-01-31 --> 6 years 10 months 1 day 2019-01-31 to 2012-04-30 --> 6 years 9 months <---- April (any day in April) 2012-04-30 to 2019-01-31 --> 6 years 9 months 1 day 2019-01-31 to 2012-05-01 --> 6 years 8 months 30 days 2012-05-01 to 2019-01-31 --> 6 years 8 months 30 days -- non-leap year 2013: 2019-01-31 to 2013-04-30 --> 5 years 9 months <---- April (any day in April) 2013-04-30 to 2019-01-31 --> 5 years 9 months 1 day I stumbled upon it while testing some JavaScript time and date frameworks and wanted to use Wolfram|Alpha because I was somewhat confused with the correct interval between two dates.

mjevans 5y ago
Try your test cases in other languages. E.G. python3 import datetime ; (datetime.datetime.strptime("2019-01-31","%Y-%m-%d") - datetime.datetime.strptime("2012-02-29","%Y-%m-%d")) The result, which reminds me of one of the other reasons I avoid python for trivial projects, is the duration / interval answer of days=2528. Part of the bug is surely in Wolfram|Alpha returning the interval broken out in human durations; but importantly those durations __don't use fixed time values__. The precise length of a //year// and of a //month// are variable. I still suspect there's double or single counting of leap-days in those durations as the reverse ones clash with converting a duration back to a whole number.

mjevans 5y ago

Try your test cases in other languages. E.G. python3 import datetime ; (datetime.datetime.strptime("2019-01-31","%Y-%m-%d") - datetime.datetime.strptime("2012-02-29","%Y-%m-%d")) The result, which reminds me of one of the other reasons I avoid python for trivial projects, is the duration / interval answer of days=2528. Part of the bug is surely in Wolfram|Alpha returning the interval broken out in human durations; but importantly those durations __don't use fixed time values__. The precise length of a //year// and of a //month// are variable. I still suspect there's double or single counting of leap-days in those durations as the reverse ones clash with converting a duration back to a whole number.