Those are harder to get wrong, since leap year bugs are almost always from consumers of dates. To screw up a not-a-leap-year-year you'd need to produce a bogus

Those are harder to get wrong, since leap year bugs are almost always from consumers of dates. To screw up a not-a-leap-year-year you'd need to produce a bogus February 29 on a real March 1. Date producers tend to understand how the calendar works better than that.