How can I have a look-up field that grabs the hourly salary for an employee as it was on the date that employee worked?

I would float up the most current rate and its effective date along with the previous rate and it's efefctive date to the parent record using a reverse relationship.  Then look that up down to the time entries and then use the snapshot feature to freeze that data.
http://www.quickbase.com/user-assistance/Default.html#setting_up_snapshot_fields.html

Then down at the time entries I would select the snapshot which applied to the date worked.  That way the user could preload salary changes ahead of time.

Post back if you need need help with the reverse relationship setup.