I noticed this a long time ago but disregarded it and chalked it up to mod issues/save problems/whatever, but now I'm absolutely certain: the way the game keeps track of seasons is flawed. I did some searching and found user and modder rezecib explaining this:
This seems to be an accurate assessment of the issue; I started looking through both the game's code and the code for the season clock in rezecib's popular "Combined Status" mod, and I saw neither evidence that a season should ever last an extra day by design (the wikis incorrectly claim that the SW seasons beyond mild each last an extra day; this is probably a combination of observing the inaccuracy and grepping the SW code for season lengths, which may lead someone to a commented-out chunk of code indicating such), nor that the mod itself could be causing it or misreporting season information; it pulls all its information from the game's builtin season manager component.
Steps to Reproduce
Play the game you made.
This issue has not been confirmed by a developer yet.