From Ring of Brodgar

Seasons switch data

Season case Remark
Autumn (Y0) ~ Aug 21, 2019 9:13 pm (Patch post date).
Winter (Y0) ~ Sep 1, 2019.
Spring (Y0) < Sep 10, 2019 7:49 am (presuming UTC/GMT) (Spring HnH Topic: ship-doc issue bug post date)
Assumed best date-time season-calibration-point at this moment: 2019-09-10 07:00:00.
Summer (Y0) ~ Sep 20, 2019.
Autumn (Y1) ~ Oct 16, 2019 Seems its a bit early (for older season-dates table). Based on post ...

Season switching (general)

I don't know if the old legacy globtime data is available inside Hafen custom clients.
If it is it would be helpful to know the globtime in relation to season-switching related gfx-changes.
The main target here would be the changing of the in-game day/night graphical-clock (as easy to spot change).
Note that season-switching could be a more or less a complete soft process.
Ie: Game settings (not just gfx stuff) could be slowly changed from current season-values to the next season-values. Making season switching a time-duration process.
As such, additional time info on other season-switching graphical-changes could show some light on the potential season-switching duration.
(Note: This in-game checking related to season switching is not something I will look into (at least not in the near future). Ergo: Someone else will need to look into this).
--.MvGulik. 03:02, 24 October 2019 (EDT)