=DATE(YEAR(A2),MONTH(A2)+3,DAY(A2))
or
=EDATE(A2,3) [check Excel help if you get a #NAME! error from this one]

Usual notes of caution apply, such as the question as to what you regard as
being 3 months after 30th November. The 2 formulae above give 2 different
results.
--
David Biddulph

"Felicity" wrote in message
...
I have a row of different date when people joined a study. I need to
calculate the exact date three months afterwards. How do I create a
formular,
so I don't have to calculate it separately for each different person(given
that thye joined on different dates?