What do you want, to advance 3 months or a year like you state in your
subject line

If you have a date in A1 then you can add 3 months like this

=DATE(YEAR(A1),MONTH(A1)+3,Day(A1))

do the same with a year

=DATE(YEAR(A1)+1,MONTH(A1),Day(A1))

now depending on the day of the month you can get unexpected results for
instance if the date you want to add 3 months to is 11/30/07 then the date
will come out as 03/01/08. There are remedies for this if you want the last
date of February instead.

=MIN(DATE(YEAR(A1),MONTH(A1)+{3,4},DAY(A1)*{1,0}))

Depends on "exactly" what you want as a result..........

This, copied down column A  would give equal time periods of "about 3 months"

=A1+365.25/4

Vaya con Dios
Chuck, CABGx3