I often have pre-show live announcements, and if that's the case, my first line is Preshow Announcement Begins (Allison) 8:01, then Act One Begins 8:04, with Allison being the speaker du jour. That way they can also tell how long the person has talked, and your run time reflects the actual show, not the preshow speaker.
I guess I don't usually put when we open the house unless it's anything very different from the standard half-hour.
Erin