as far as I know, SQL does only keep track of creation dates of tables and not for data modification. Therefore I think you should use a last_modified column of datetime and triggers to update this column.
A modified_datetime field (which you will need to update using a trigger on each table) is the preferred method. I usually store two values; the modified date and the modifier, which I pull from one of the system user nyladic functions.
If you can't modify the database, there is probably a way to do it (perhaps through a third-party application) by monitoring the log.