今天看了报纸才知道的.原来根据现在用的gregorian calendar,并非被4整除就是闰年.还有另外条件：Every year that is exactly divisible by four is a leap year,except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years.（from wiki）.所以说,1700,1800,1900,2100,都不是闰年,虽然他们都能被四整除.而2000年因为能够被400整除,所以仍旧是闰年.闰年比平年多一天,2月分有29天,全年一共366天