The inconsistency with GNU date is more visible from Feb: $ date -d "jan 31 next month" Wed Mar 3 00:00:00 PST 2021 $ date -d "jan 30 next month" Tue

The inconsistency with GNU date is more visible from Feb: $ date -d "jan 31 next month" Wed Mar 3 00:00:00 PST 2021 $ date -d "jan 30 next month" Tue Mar 2 00:00:00 PST 2021 $ date -d "jan 1 next month" Mon Feb 1 00:00:00 PST 2021 $ date -d "feb 28 next month" Sun Mar 28 00:00:00 PDT 2021 Edit: This was on my unconscious mind for a bit and I came up with an additional test case to confirm a suspicion I realized. $ date -d "2016-1-31 next month" Wed Mar 2 00:00:00 PST 2016 date -d "2016-2-1 next month" Tue Mar 1 00:00:00 PST 2016 date -d "2016-2-1 next year" Wed Feb 1 00:00:00 PST 2017 $ date -d "2016-3-1 next year" Wed Mar 1 00:00:00 PST 2017 GNU date will add the duration of the CURRENT interval (ignoring already occurred deviations, like leap years) relative to the specified base date. The oddity in behavior I observed above is adding the length of the month of Jan to dates in Jan. I suspect only a programmer would find that inference remotely correct.