the time at which something is supposed to begin
"they got an early start"
"she knew from the get-go that he was the man for her"
The time at which an entertainment event is set to begin.
"The crowd started cheering loudly as showtime approached and the lights dimmed for the opening act."
A declaration that something is about to happen, particularly that the person speaking is about to act.
"She stepped up to the microphone and declared showtime before launching into her surprise song."