You're right. Most random number functions rely on a "seed" and that seed usually comes out of the system clock. However, even if you could reverse engineer the function to predict the number, it's still nearly impossible to guess a randomly generated password since for that, you'd have to know the time at which it was generated down to the last millisecond. Finding that out is nearly impossible, since there are 60000 milliseconds in a minute, 3600000 in an hour and 86400000 in a day. Needless to say, that's a hell of a lot of possible seeds.

Cheers,
cgkanchi