February 29th, 1991 is not a valid date. The last leap year before 1991: 1988. The next leap year after 1991: 1992. Details below.

The leap year algorithm. The three conditions to check in order to know whether a year is a leap or a common one:

Every year that is evenly divisible by 4 is a leap year, unless they are evenly divisible by 100, in which case they also need to be evenly divisible by 400.

1) A leap year can be evenly divided by 4:

1991 is not evenly divided by 4.

2) Except if it can be evenly divided by 100 then it isn't a leap year:

1991 is not evenly divided by 100.

3) Except if it can be evenly divided by 400, then it's a leap year:

1991 is not evenly divided by 400.

1991 is not a leap year, it is a common year. What is a leap year?

A leap year has 366 days. 1991, being a common year, has 365.

In a leap year month February has 29 days. February 29th is a valid date.

In a common year month February has 28 days. February 29th, 1991 does not exist.

How often do the leap years occur?

Nearly once every four years is a leap year. More exactly, leap years occur 97 times in every cycle of 400 years.

Why do we need leap years in our calendar?

Leap years keep the calendar year in sync with the astronomical year, preventing us, for example, from celebrating Christmas in the month of November.

The last leap year before 1991: 1988. The next leap year after 1991: 1992.

Is it a leap year? Last leap year before & next leap year after this year

The leap year algorithm. The three conditions to check in order to know whether a year is a leap or a common one:

1) A leap year can be evenly divided by 4

2) Except if it can be evenly divided by 100 then it isn't a leap year

3) Except if it can be evenly divided by 400, then it's a leap year

Every year that is evenly divisible by 4 is a leap year, except for years that are evenly divisible by 100, but these years are leap years if they are evenly divisible by 400:

Examples of leap years.

Leap years multiples of 4 but not of 100 nor of 400: 1992, 1996, 2004, 2008, 2012, 2016, 2020, 2024, 2028;

Common years (NOT LEAP YEARS) multiples of 100, but not of 400: 1700, 1800, 1900, 2100, 2200, 2300;