Nowadays, the world's common calendar, not leap in February for 28 days, a year of 365 days; every four years there is a leap year, in the leap year in February for 29 days, the whole year for 366 days. Set the leap year regulations: the Gregorian calendar for the standard, all can be divided by 4 years is a leap year; but in the year of 100, can be divided by 4 is not necessarily a leap year, must be able to be divided by 400 is a leap year. For example, 1968 is a leap year if it is divisible by 4. 1900 is a leap year if it is divisible by 4 but not by 400, so it is not a leap year; 2000 is a leap year if it is divisible by 4 but also by 400. 2011 is not a leap year if it is not divisible by 4, so it is not a leap year, and so the month of February in 2011 is 28 days.