This is most commonly done with a sheet-level macro. The macro would watch cell A1 and when a value is entered there matching your criteria, the macro puts a datestamp into another cell of your choosing. Is that OK?

If so:
1) What cell should receive the datestamp?
2) Is this a row-by-row function? Meaning if A1 changed, B1 gets a date, A2 >> B2, A30 >> B30 ...etc? Or is this just a one-cell thing?
3) What if someone CHANGES the value that is in A1 after a datestamp had already been created? Do you want the datestamp updated?