https://www.wolframalpha.com/input?i=2016-04-04+to+2019-01-3... = 2 years 9 months 27 days
https://www.wolframalpha.com/input?i=2019-01-31+to+2016-04-0... = 2 years 9 months 26 days
The duration should be identical but is off by one day.
One day a space mission will fail due to this bug.
Reported several times, they never cared.
both show an identical duration of 2 years 9 months 26 days (edit: despite reporting 1032 and 1031 days respectively).
Months are lunar. Days are solar.
Jan 2024 - Nov 2023 example (both cases show "2 months 1 day"):
https://www.wolframalpha.com/input?i=2024-01-31+to+2023-11-2...
https://www.wolframalpha.com/input?i=2024-01-30+to+2023-11-2...
Aug 2024 - Jun 2024 example (same bug):
https://www.wolframalpha.com/input?i=2024-08-31+to+2024-06-2...
https://www.wolframalpha.com/input?i=2024-08-30+to+2024-06-2...
The same thing doesn't happen from Jul 2024 - May 2024 (results vary by 1 day):
https://www.wolframalpha.com/input?i=2024-07-31+to+2024-05-2...
https://www.wolframalpha.com/input?i=2024-07-30+to+2024-05-2...
Date time arithmetic is weird ;)
https://www.wolframalpha.com/input?i=2019-01-31+to+2015-10-2... = 3 years 3 months 10 days
https://www.wolframalpha.com/input?i=2015-10-21+to+2019-01-3... = 3 years 3 months 10 days