Date and Time
Determines whether a year is a leap year.
If yes, the function will return the value 1 (TRUE); if not, it will return 0 (FALSE).
Date specifies whether a given date falls within a leap year. The Date parameter must be a valid date.
=ISLEAPYEAR(A1) returns 1, if A1 contains 1968-02-29, the valid date 29th of February 1968 in your locale setting.
You may also use =ISLEAPYEAR(DATE(1968;2;29)) or =ISLEAPYEAR("1968-02-29") giving the date string in the ISO 8601 notation.
Never use =ISLEAPYEAR(2/29/68), because this would first evaluate 2 divided by 29 divided by 68, and then calculate the ISLEAPYEAR function from this small number as a serial date number.