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)

Update "Smoked Help" season changes

As of the update mentioned (see, the total times for seasons have been given as Spring=10, Summer=15, Autumn=10, Winter=5 (real days). The start of the year (Spring) on the calendar is currently accurate (as long as the data is synched with the server), but the start dates of Autumn and Winter will remain off until the code that generates the calendar can be fixed. --MagicManICT (talk) 04:22, 31 July 2020 (UTC)

Season Calendar (outdated)

Season Start ISO (UTC)
Spring Tuesday, 04 August 2020 2020-08-04 22:02:36
Early Summer Friday, 14 August 2020 2020-08-14 02:54:08
Mid Summer Sunday, 23 August 2020 2020-08-23 07:45:40
Late Summer Tuesday, 01 September 2020 2020-09-01 12:37:13
Autumn Thursday, 10 September 2020 2020-09-10 17:28:45
Winter Saturday, 19 September 2020 2020-09-19 22:20:17
Spring Tuesday, 29 September 2020 2020-09-29 03:11:49
Early Summer Thursday, 08 October 2020 2020-10-08 08:03:21
Mid Summer Saturday, 17 October 2020 2020-10-17 12:54:53
Late Summer Monday, 26 October 2020 2020-10-26 17:46:25
Autumn Wednesday, 04 November 2020 2020-11-04 22:37:57
Winter Saturday, 14 November 2020 2020-11-14 03:29:29
Spring Monday, 23 November 2020 2020-11-23 08:21:01
(Hit MW-page refresh button if pink time is later than NOW)
Note 1: The dates in this calendar are predictions. There accuracy is still considered a work in progress. (assumes season switch at start of new moon)
Note 2: Potential Game-server clock/time changes are not taking in account here. (Down time, Rollback, etc. Rare now a days).
Current Season calibration point: Friday, 21 February 2020 (ISO:2020-02-21 06:35:00) Season: Spring.