What factor is used to determine a century leap-year?

Answer 1

Only century years divisible by 400 qualify to be a leap year. Years divisible by 4 but by 100 are NOT leap years, thus 1700, 1800, and 1900 were not leap years but 2000 was. 2100, 2200 and 2300 will not be leap years but 2400 will be.

Answer 2

A century leap year is a year that is evenly divisible by 400, like 1600, 2000, 2400, etc. Years that are evenly divisible by 100 but not evenly divisible by 400 are not leap years.