The "One Day Only" pricing has been around for awhile but recently converted to "Today Only," likely for clarification purposes. The "One Time Only" that refers only to pricing for a specific show has been in place for just a month or so; it seems like a good idea to clear things up early in the game.
Interestingly, you can also search for offers on the website under "One Day Only," "Today Only," "One Time Only," and similar phrases throughout each day, so it would probably be a good idea for QVC to do some overall cleanup on its terminology. (All the problems with the site's search functionality are a task/topic for another day!)
ETA: @elated - It appears that you are proving my point; the change in terms and offer names is already causing some confusion.