If you add too many days to the Date object, it will automatically roll over the month (or even the year) for you.
date = new Date("May 29, 2016");
// Sun May 29 2016 00:00:00 GMT+1200 (New Zealand Standard Time)
date.setDate(date.getDate() + 5);
// 1464868800000
date.getDate();
// 3
date;
// Fri Jun 03 2016 00:00:00 GMT+1200 (New Zealand Standard Time)