How to save the time the server was first started?/Make a second world seed?

I'm trying to save the time that the server first starts/track the time since the server first started. Ticks won't do it, because they stop when the server is removed, and can be modified by setting the time. So how do I create my own seed, something that won't change every time the server is rebooted?

I have a feeling that the game actually does have it in there somewhere (possibly long k in the dedicated server class?), but I need to keep it in a specific format (whatever the System.currentTimeMillis( ); returns). Plus, I can't use the seed if it's customized, it would throw everything off. BUT--in the event that this is a usable time, how on earth would I call it from my mod?

Also, I'm aware that calling for the system time might screw things up if they change their computer time to before the start of the server. I already have a safeguard for that.