How leap years are calculated

In the Gregorian calendar a year has 366 days if it is divisible by 4, except century years, which must also be divisible by 400. 2000 was a leap year; 1900 was not; 2100 will not be.

Date tools on this site reject 29 February in common years. Adding one month to 31 January lands on 28 February, or 29 February in a leap year.