要求闰年二月为29天，方法越简便越好……
闰年规则是：
普通年能被4整除且不能被100整除的为闰年。（如2004年就是闰年,1901年不是闰年）
世纪年能被400整除的是闰年。(如2000年是闰年，1900年不是闰年)
闰年规则是：
普通年能被4整除且不能被100整除的为闰年。（如2004年就是闰年,1901年不是闰年）
世纪年能被400整除的是闰年。(如2000年是闰年，1900年不是闰年)
alert(new Date('2009/03/00').getDate())
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已经区分好闰年的