the comment sends you to a rather complicated serie of codes... there is a far more simple way to get that, here is the code :
function test() {
Logger.log('today= '+new Date()+' and 5 days ago is '+subDaysFromDate(new Date(),5));
}
function subDaysFromDate(date,d){
// d = number of day ro substract and date = start date
var result = new Date(date.getTime()-d*(24*3600*1000));
return result
}
Logger result :
[13-11-18 23:39:50:364 CET] today= Mon Nov 18 2013 23:39:50 GMT+0100 (CET) and 5 days ago is Wed Nov 13 2013 23:39:50 GMT+0100 (CET)
if you want to get the date in the form dd/mm/yyyy use Utilities.formatDate(date, timeZone, 'dd/MM/yyyy)
, see doc here