How to subtract days from a plain Date?

Try something like this:

 var d = new Date();
 d.setDate(d.getDate()-5);

Note that this modifies the date object and returns the time value of the updated date.

var d = new Date();

document.write('Today is: ' + d.toLocaleString());

d.setDate(d.getDate() - 5);

document.write('<br>5 days ago was: ' + d.toLocaleString());

var dateOffset = (24*60*60*1000) * 5; //5 days
var myDate = new Date();
myDate.setTime(myDate.getTime() - dateOffset);

If you're performing lots of headachy date manipulation throughout your web application, DateJS will make your life much easier:

http://simonwillison.net/2007/Dec/3/datejs/

It goes something like this:

var d = new Date(); // today!
var x = 5; // go back 5 days!
d.setDate(d.getDate() - x);