Assuming you have the following two fields, dtDate (Date, e.g. 6/12/2006) and dtTime (9:00:00 PM), you can do the following.
dtCombine: DateSerial(Year([dtDate]),Month([dtDate]),Day([dtDate])) +
TimeSerial(Hour([dtTime]),Minute([dtTime]),Second([dtTime]))
There may be a better way...