DatePlus

DatePlus[date, n]
finds the date n days after date.
DatePlus[date, {n, “unit”}]
finds the date n units after date.
DatePlus[date, {{n1, “unit1”}, {n2, unit2}, ...}]
finds the date which is n_i specified units after date.
DatePlus[n]
finds the date n days after the current date.
DatePlus[offset]
finds the date which is offset from the current date.

Add 73 days to Feb 5, 2010:

Add 8 weeks and 1 day to March 16, 1999: