While a normal year has 365 days, a leap year has 366 days. Since it takes the Earth about 365 days, 5 hours, 48 minutes, and 46 seconds (equivalent to 365.242199 days) to make one circle around the Sun, a leap year is needed to keep our calendar in alignment with the Earth's revolutions around the sun.

The leap year occur every 4 years, so one of the basic rules to check if a year is a leap year is to see if the year can be evenly divided by 4. For example, year 2012 is a leap year because 2012/4=503, i.e. evenly divided.

If a year is evenly divided by 4 but at the same time can be divided by 100, it is not a leap year, for example, year 1900. However, it can be divided by 400, it is a leap year, e.g. year 2000.