o'clock

I recently encountered the phrase pattern "[activity noun x] o'clock" meaning "it's time to do x": once as "sex o'clock" (link is not work-safe), once as "pray o'clock" (in an unpublished story, but the phrase also appears on various web pages).

Googling turns up other phrases with this pattern: "work o'clock," "study o'clock," "dinner o'clock," and so on.

I like it--fun phrase. (Especially "sex o'clock" because it's almost a pun.) I wonder how long this has been around--anyone know? I had never seen it before about a week ago. Though it's similar to other non-numeric time identifiers, like the classic military "oh-dark-hundred."