One way:
=Date(YEAR(A1),MONTH(A1)+6,DAY(A1))
another:
=DATE(YEAR(A1),MONTH(A1)+6, MIN(DAY(A1), DAY(DATE(YEAR(A1),
MONTH(A1)+6+1,0))))
Note, however, that "months" is a rather imprecise term - is one month
after 31 January = 28 February, 29 February, 2 March? (the first formula
above, using 1 month instead of 6, produces 2 March for 2004, the
second, 29 February).
Is one month after 30 January (say 29 February) = one month after 31
January?