If the file is hosted from FileMaker Server, you may be able to set up a schedule that runs a script at 8PM. But there are limits to what a server side script can do with imports: http://help.filemaker.com/app/answers/detail/a_id/7035/kw/server%20import%20records
Running the same script client side, a script can use InstallOnTimerScript with a calculated interval that calculates the interval from when the script was performed (prehaps when it was opened) to 8 pm. This interval has to be in seconds.
Install OnTimerScript ["importScriptHere" ; Interval: GetasTime ( "8:00 pm" ) - Get ( CurrentTime ) ]