What year does Senran take place in?

As far as I'm aware/remember there is no set year, though 2011 feels like it would be a safe guess.

2012 could work too, yeah.

Hibari's age in EV was likely a mistake, since other characters who had birthdays between SV and EV had their ages listed appropriately. I don't know what their ages are listed as in SK2, but a decent portion of the game's final chapter takes place on August 16th, indicated by the Daimonji Bonfire that can be seen in the background of the cutscenes.

If you want to get even more specific, it's likely that Burst starts around late June at the earliest. Hanzo Academy is explicitly located in Tokyo, and Hebijo probably is as well. Both stories have a scene where the girls go to a beach, and the official swimming season for beaches around Tokyo starts around mid-July. This also lines up with SV taking place 6 months later.

That said, I double-checked and I was wrong, all of the ages listed are the same as they are in SV. Not sure where I got that impression from. So, either they were lazy and got the ages wrong, or EV takes place mere days after SV. It does make some sense; Katsuragi, for example, could've been so excited by their sudden beach trip because it was still in the middle of winter. Because Yumi's birthday is on New Year's Eve, the Ryou-twins' are on January 19th, and their ages are listed the same in both SV and EV, that would point to both games' main stories taking place between those two dates. Strictly speaking, iirc, there's nothing that points to Hibari's birthday being directly tied to the events of SV, nor anything that really suggests it couldn't be after EV, since it was just a side-story in SV...

I personally think it was probably just laziness and not paying attention, but since I can't really think of anything that disproves it, I guess SV and EV taking place in the same 2 1/2 week timeframe is my current theory.