IIRC there was a way where you had a stealth ship moving at say 100m/s a ways away from the action area. Every 10 meters it traveled, 0.1 seconds went by, and that's how it would be measured.
like this:
When ( "Ship-has-traveled-meters (or correlating SEXP)", 10)
{
event
}
When ( "Ship-has-traveld-meters", 15)
{
event 2
}
When ( "Ship-has-traveld-meters", 21)
{
event 3
}
Actually, I think I screwed up the general method, but I don't really know how to re-phrase it without mucking around in FRED for a half an hour, so I hope you get the type of thing I'm saying.
EDIT: And this seems like it would be one of those hack-ish methods. Sorry I couldn't help. :/
EDIT2: I hate you. Now I'm mucking around in FRED.