The calendar that we use, the Gregorian calendar, is intimately related to two astronomical periods: the day, the mean time between two noons and the year, the time for the earth to go around the sun once. The ratio between these is not an integer, so we need to introduce a leap year. What happens on other planets? Are there different definitions of leap years on earth which would be more accurate? This IB project discusses how we can define calendars for other planets.