Don't believe this is always possible due to the "Bad Stuff Happens" factor. Last test we blew it up. Not much choice to shut it down. Also hard to predict how long it will take to find the wily buggies that were discovered and implement fixes for them. Welcome to Beta :)
Opening times should be an estimated time posted the day before. Closing times should be an estimated time, posted at least the day before. Last test went on longer than it was supposed to, though I bet people think it ended early because of the bug/reboot.
If estimated opening and closing times were posted, there would be a lot more unhappy people. People unhappy because the game opened before the posted time. People unhappy because the game didn't open at the posted time. People unhappy because the game closed sooner than the posted time. People unhappy because the game went on after the posted time.
Beta testing is not for players. Beta testing is not to "do us all good". Beta testing is for developers to learn what is working and not working with the code they have written. There is no benefit to developers to fit their schedules around our expectations, and there is certainly no benefit for them to encourage our expectations by posting a meaningless time that will only cause unhappiness.