Not as wonky as you'd think, and 60 minutes means something much different than 12:15.

The expression you’ve posted is literally a complicated way of writing 60mn, and you specifically wrote that it should not be interpreted as 12:15.

See also ETags which stop trying to be clever about dates

Etags have unrelated role and semantics. Etags go with last-update, which doesn’t suffer from anywhere near as much from drift because it’s checked by the server, which emitted it.