you can probably have a script that calculates the time the new clients logon,
and have it forward or start the movie clip based on the time they login.
so if the movie is say 3 hours long, and started at 10:00am and the new visitor logs in at 10:25am, it would calculate him as 25 minutes late, and start
the movie playing from 25 minutes onwards.
i'm not certain how to start the movie at any point, but i'm pretty sure it can be set to start at any point in the movie.