I had a fun bug once where some functionality involving self sign certificates stopped working on Feb 29th. It came down to us simply adding +10 to the year for the expiration. There is no Feb 29th 10 years into the future, only 4, 8, 12, etc... That was a good lesson on why to always use a date library when adding or subtracting time. We weren't the only ones... Azure went down for the same reason that day.