Maybe this which calculates the number of 90 day periods that have elapsed
since the start date.

=(FLOOR((A1-TODAY())/90,1)+1)*90+A1

Understand though that working with dates is tricky. There is a big
difference between 90 days, 3 months and one quarter of a year so if you
weren't looking for exactly 90 day intervals this probably won't meet your
needs.

PC
"sonoundio" wrote in message
...
I need to be able to set set a deadline based on prior dates. These can
go
back years. I would like to set up a current 'next appointment, for
instance
ever 90 days, based on a date of arrival. For example, if date of arrival
is
2/13/03, how do I make a formula that is effective now. In other words,
the
cell updates to a new deadline every ninety days, and stays current.
Thanks a bunch.