GP - TOTW - 28

There are 1461 days between 29 February one leap year and the same date the next leap year: This is what a perpetual calendar takes into account.

The perpetual calendar mechanism has a mechanical memory providing a date display which automatically takes into account the different month lengths including those in leap years. A mechanism based on a so-called "forty-eight cam' completes one revolution every 4 years to take into account February 29th every leap year. However, perpetual calendars will need to be corrected by one day on March 1st 2100. This is not due to a mechanical defect, but because under the Gregorian calendar the year 2100, like three of every four centuries, will not be a leap year.