It's 2401, confirmed by Terry Matalas and the stardate given in "No Win Scenario". If they'd made Frontier Day the 250th anniversary of, say, the signing of the Coalition of Planets charter, bumping it up to c.2405, there'd be no issue.
So season 3 takes place in the same year as season 2? Because season 2 (the premiere and finale 25th century scenes) were also in 2401.
Also, prior to the season airing Terry had said there would be a time jump of a few years between seasons 2 and 3..sounds like he later retracted that statement