I ran into this already on January first. Someone in the last 4 years changed/created a function in a MUD that I now maintain, to convert Unix timestamps into iso8601 strings for MySQL.
They did the math for which year is a leap year incorrectly, which wasn't the bug that bit us, they helpfully added the leap day regardless of where in the year it was. The tests they wrote targeted the middle of the year and missed it. I just pulled in date2j and related match from postgres, added more test cases.
I found this out the hard way as well, at some point since I last logged in they quietly enacted a policy of deleting keys that aren't used for roughly a year.[1] This meant I couldn't use the not so public method of verifying I still had access to various associated keys.
They did the math for which year is a leap year incorrectly, which wasn't the bug that bit us, they helpfully added the leap day regardless of where in the year it was. The tests they wrote targeted the middle of the year and missed it. I just pulled in date2j and related match from postgres, added more test cases.