Hi again,
If you create a new (blank) Access database it is a trivial exercise to
import all of the items from another database, so there's no real way to
check if it's their own work or not (at least using the dates). Try copying
the database in Windows Explorer - this will also change the file dates.
I guess if you think the students aren't that clever, you could try putting
a hidden table in each base database and check for its existence in the
submitted database... still no guarantee though...