Area 51 sets a fixed time largely for historical reasons. Note that it's never actually been accurate - even when sites launched around 19:00 UTC, launches are a manual process and are never precise. (Come to think of it, it also doesn't account for daylight savings - launches used to be timed to around 15:00 Eastern, which turns into 19:00 or 20:00 UTC, depending on the time of year.)
These days we try to launch closer to morning Eastern time, but once in a while things get in the way - designers are backlogged and don't get to make the icons, I sleep in another hour, systems are under maintenance, etc. etc. This may even change to another day in the week at some point, who knows.
We could have it record the exact time. I don't really know off-hand if that'd break anything that relies on a fixed timestamp. Probably not. But since doing that would not actually fix the core issue of wanting to jump in on a new site early, there isn't much point in digging further into it.
We send out emails when a site goes into private beta. I suggest using that to time your activity.
Last but not least, no site is made or broken by the questions posted or tags created within the first few hours of a private beta. You have days to create a great site. Take your time. (But that userid 1 is mine.)