What should happen if it's not? Should the value be rejected, or just show an error message?
What is the date format of that field?
Dates are more complicated than regular calculations because Dates are more complex.
You can read about scripting for Date Handling here:
There are two important bits for such a script
1) Parsing the entered date text into a Date object
2) Adding 90 days to the current date so the date objects can be compared.