Leap-Day Span Calculator

Worked example

2028 is a leap year (divisible by 4, not a century year), so it has a February 29. 2100 is not a leap year, despite being divisible by 4, because century years need to be divisible by 400 too, and 2100 isn't. 2000 is a leap year, since it is divisible by 400. From January 1, 2027 to January 1, 2029, exactly one leap day falls in range, namely February 29, 2028.

Why century years are the exception

A simple "every 4 years" rule drifts the calendar out of sync with the solar year by about 3 days every 400 years. Dropping the leap day in century years not divisible by 400 (like 1900 and 2100, but not 2000) corrects that drift, which is why 2000 felt like an exception to an exception.

Common mistakes

• Applying the "every 4 years" rule without checking the century exception. A year divisible by 4 is a leap year unless it is also a century year, in which case it needs to be divisible by 400 as well, not just 4.
• Assuming every century year skips the leap day. Only century years not divisible by 400 skip it. 2000 kept its February 29 for exactly this reason, while 1900 and 2100 do not.

Limitations

This identifies leap days for calendar purposes only. It is not a legal or actuarial determination for a February 29 birth date, and it does not address how a specific jurisdiction or contract handles an anniversary that falls on a leap day.