We're tracking a Facebook bug in which scheduled posts to a Facebook Business Page may appear to "go live" earlier than they were scheduled for.
If you see your post on your Page before it's supposed to be there, don't panic! Chances are you're affected by this bug. Only admins see the Post early - your regular users won't see it in their feeds or on the Page until the correct time.
To double-check, you can go to your Page on Facebook.com, click "Publishing Tools" up the top, then "Scheduled Posts".
If you see your scheduled post there, you can rest easy - it's set up properly.