Does anyone know how to set up a data validation in a cell which contains a formula, and the only way to change the cell value (or at least warn the user) is to input the same formula (with certain modifications)?

For example

If the cell was to calculate a time based on a particular cell plus or minus x hours or minutes, I would use the formula

D3 + TIME(1,0,0)

so that the cell would give the value of D3 plus 1 hour.

Therefore, the cell data validation would only allow a formula which was in the same format.

ANy ideas?

Tahnks