asp: timestamp updating

I have created an online test. I need to have the
timestamp field update automatically when the users submit
their answers, as they can come back many times to
complete the test. I have the appropriate fields in the
database, but have not been able to figure out how to have
it update without the user entering the date (I don't want
to rely on them to be completely honest). How do I enter
this into my sql stmt., or elsewhere, ....???? Please
help, I am under a deadline.