js 日期的加减

Date.prototype.Format = function(fmt) 
{
//author: meizz 
var o =
{ 
"M+" : this.getMonth() + 1, //月份 
"d+" : this.getDate(), //日 
"h+" : this.getHours(), //小时 
"m+" : this.getMinutes(), //分 
"s+" : this.getSeconds(), //秒 
"q+" : Math.floor((this.getMonth() + 3) / 3), //季度 
"S" : this.getMilliseconds() //毫秒 
}; 
if (/(y+)/.test(fmt)) 
fmt = fmt.replace(RegExp.$1, (this.getFullYear() + "").substr(4 - RegExp.$1.length)); 
for (var k in o) 
if (new RegExp("(" + k + ")").test(fmt)) 
fmt = fmt.replace(RegExp.$1, (RegExp.$1.length == 1) ? (o[k]) : (("00" + o[k]).substr(("" + o[k]).length))); 
return fmt; 
}
Date.prototype.addDays = function(d)
{
this.setDate(this.getDate() + d);
};
Date.prototype.addWeeks = function(w)
{
this.addDays(w * 7);
};
Date.prototype.addMonths= function(m)
{
var d = this.getDate();
this.setMonth(this.getMonth() + m);
if (this.getDate() < d)
this.setDate(0);
};
Date.prototype.addYears = function(y)
{
var m = this.getMonth();
this.setFullYear(this.getFullYear() + y);
if (m < this.getMonth()) 
{
this.setDate(0);
}
};
var now = new Date();
now.addDays(1);//加减日期操作
alert(now.Format("yyyy-MM-dd"));