A potential problem for some systems that follow an erroneous algorithm for calculating leap years. There are three rules for calculating leap years: (1) A year is a leap year if it is divisible by 4, but (2) not if it is divisible by 100, unless (3) it is also divisible by 400. Thus, 1900 was not a leap year, but 2000 was. In the Gregorian calendar: any year divisible by 4 except centenary years not divisible by 400; SYN. intercalary year, 366 days, bissextile year. leap-year