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());