On the toolbar, go to Data | Validation while you are on the cell containing
date the User is entering (in my example, cell B1). Select "Custom". In the
Formula box type:
=IF(B1-A1>90,1=2,1=1)
Where B1 = the cell the user is data entering
Where A1 = the existing date
The formula is say if the date (in B1) is more than 90 days from the date
(in A1) then 1=2 (which is false and will give the user an error message).
If the date in B1 is less than the date in A1 then 1=1 (which is true & no
error message given to user). Play around with it to make it fit your
situation. Also, look at the other tabs, you can customize the error
message, etc.
See also:
http://www.contextures.com/xlDataVal01.html
Thx
MSweetG222