Also remember the dreaded 26-day (or something) wrap-around of the millisecs counter. Not that I expect anyone running the computer 26 days straight and playing my game without rebooting, but that's always a possibility, especially when used in game servers.
For that reason I feel using a real timer is a bit more foolproof, although you can implement additional checks for detecting the millisecs wrap.
For that reason I feel using a real timer is a bit more foolproof, although you can implement additional checks for detecting the millisecs wrap.