Adding/Subtracting days from a date doesn't change the year/month correctly

Question

If I have a date that is 2011-01-02 and I subtract 7 days from that date it should give me 2010-12-26, but instead it gives me 2011-01-26?

See the JS below to verify with link:

var date = new Date('2011','01','02');
alert('the original date is '+date);
var newdate = new Date(date);
newdate = newdate.setDate(newdate.getDate() - 7);
var nd = new Date(newdate);
alert('the new date is '+nd);

http://jsbin.com/upeyu/6

Answer 1

I think you meant to do this: (working perfectly)

var date = new Date('2011','01','02');
alert('the original date is '+date);
var newdate = new Date(date);
newdate.setDate(newdate.getDate() - 7);
var nd = new Date(newdate);
alert('the new date is '+nd);

jsFiddle example

Answer 2

getDate() and setDate() both refer to only the day of the month part of the date. In order to subtract 7 days you want to do this:

myDate.setDate( myDate.getDate() - 7 );

This sets the day of the month to the day of the month minus seven. If you end up using a negative number it goes back to the previous month.