JS how to set the time to calculate a few days ago?

　　

Calculate how many days before the specific time. For example today is September 5, that 7 days ago Normal is August 29.

Before the direct use of time to add and subtract, eat a big loss, and then brain residual to directly write a very complicated calculation leap year, leap month, 30, 31, 28 months now share.

    function Initdatetime (day) {            var nowdate = new Date ()            var agodate = new Date (nowdate);            var nowday = Nowdate.getdate ();            var day =-day + nowday;            Agodate.setdate (day);            var nowmonth = nowdate.getmonth () + 1;            var nowyear = Nowdate.getfullyear ();            var agoday = Agodate.getdate ();            var agomonth = agodate.getmonth () + 1;            var agoyear = Agodate.getfullyear ();            var datefrom = (agoyear.tostring () + '-' + agomonth.tostring () + '-' + agoday.tostring ());            var Dateto = (nowyear.tostring () + '-' + nowmonth.tostring () + '-' + nowday);            $ (': text[name=startdate] '). Val (datefrom);    Start time            $ (': text[name=enddate] '). Val (Dateto);     End Time        }

Call:

Initdatetime (7)    //calculates the time 7 days ago.

JS How to set the time to calculate a few days ago?