前天，昨天和今天

var today=new Date();
var yesterday=new Date(today.getTime()-1000*60*60*24);        
var thedaybeforeyesterday=new Date(today.getTime()-1000*60*60*24*2);        
    
var strToday=getStringFromDate(today);
var strYesterday=getStringFromDate(yesterday);
var strThedaybeforeyesterday=getStringFromDate(thedaybeforeyesterday);

 
上面用到的函数
function getStringFromDate(dd){
    var y = dd.getFullYear();
    
    var m = dd.getMonth()+1;
    m=parseInt(m,10);
    if(m<10){
        m="0"+m;
    }
    
    var d = dd.getDate();
    d=parseInt(d,10);
    if(d<10){
        d="0"+d;
    }
    
    return y+"-"+m+"-"+d;
}