Dated coins tend to be a rather slim minority type as compared to the much more common coins with actual legends rather than dates. I recently found out that dates could be both in exergue - with an actual legend up top, as well as replacing the legend. Exergual date: VIIC: PXV: Legend: And this one is neat, a dated Claudius II. These seem to be less common than Gallienus.
The first Gallienus is dated P M TR P XV P P, which corresponds to 267. He was COS VII since the previous year, thus I suppose the "VII C" in exergue means Cos. VII. But what do "P XV" mean? P for (Tribunicia) P(otestas)?
There was a decent discussion on this back in 2016. https://www.cointalk.com/threads/dated-gallienus.274426/ I don't have many dated Gallienus.
These are neat... I especially like your Claudius II. RIC online says the issue celebrates the victory at Naissus! Here's a Gallienus Serapis (Antioch), TR P XII COS V (264):