Tracer,
Normally, I give 5 - 10 minute warnings beginning at T+30 and I know I gave a '1 minute until launch' Friday. ( I think it was like 13 minutes before and 5 minutes before announcements also. )
Now, I do recall that I had to reset the game clock sometime after I had logged in, but I think that was about 40 minutes or so before launch time. It may have been sooner to launch time than that.. things were kinda rushed on my end. Normally, I have the game clock set 2 hours before the event, but couldn't be there for that this week.
Yes, you can edit and change the start time on your mission at any time ( even after you have launched! ) and even make it earlier than the wall time. All that does is immediately launch anybody who is already signed onto the mission. Just click 'yes' to the big ugly message asking if you really want to do that.