Do you mean:
The date in the cell <NOW() - 30 ? In which case you could use conditional
formatting using that as a criteria.
Or do you mean, the cell has not been changed in the last 30 days.
One possibility is to capture changes to a worksheet by hooking the _Change
event.
Then on a hidden sheet, paste the current date/time in the cell of the same
row/column.
If neither, then you'll have to clarify.
Rob