"Back in the Day"

[Douglas G. Wilson]

It seems natural, although I've never heard it AFAIK.

"Day[s]" = "time[s]" is a common usage, of course:

His day will come.
In my day, we did it differently.
Men did not exist in the day[s] of the dinosaur[s].
Those were the days.
I remember the day[s] of the black-and-white TV.
That was back in the old days. [There are no "new days", so one might as
well say "... back in the days."]

-- Doug Wilson