Hi,
First, if there are Excel legal dates in C3 then the formula =C3+90 should
return a date 90 days into the future. I'm not clear what you are actually
getting but dates don't have periods in them which suggests something else is
in C3 other than a real date, which means any formula we give you will fail.
Second, you could try changing the format of the cell with the formula to a
date format.
Third, you can get a date 3 months from any date using
=EDATE(C3,3)
that will give the date 3 months into the future from the date in C3. The
EDATE function is part of the Analysis ToolPak so you must attach it by
choosing Tools, Add-in, and putting a check beside it.
If this helps, please click the Yes button