Hey Murkalael,
I talked in another post about how we'd be re-balancing all the timed missions to have the same cool down (which I believe is slated to be 21 hours, but it's not 100% yet), however, the mission timers aren't based off of server time in a sense they reset at X time, but the timer for you to do the mission again starts once that mission is completed, does that make sense?
A dialog from the NPC that tells you come back in X time is something we've been wanting as well, although I don't know if that is possible.
I hope this helps,
Cheers,
Kris