For those who want to play with Historical Seasons on, but with non supported start dates (which are currently limited to a few expanded bookmarks, and the Vanilla AGOT Bookmarks), it is not too difficult to set up, however it will require some familiarity with either console commands if you're willing to not have achievements enabled, or with basic modding / file editing.


Steps :
1.) In the CK3 Launcher, go to mods, and select Seasons, and open local files.
a.) Open the "events" folder, open "season_events", and search for the full year of your custom start date. For example, if you want to play in 224 A.C., search for 8224.(1)
b.) You'll then see a code comment that says something like : "#Spr 8223.8.23 to Sum 8224.9.20
#394", the first date is the start date of the previous season, which in this case was spring. The next date, is the start date of the summer season, which would begin on 8224.9.20, which is 394 days after 8223.8.23, meaning that when this event fires, it'll set it to spring, and then wait 394 days until firing the next event, which will bring it to summer.

c.)Look for where it says id = season_events, you'll then copy that event id, which in this example would be season_events.099, as this is the next event in the sequence, which is directly below the current event. Keep this event ID saved.

2.) Start new game in debug_mode(2), and set Seasons to Historical.
3.) Open the console by clicking the ` key, usually near the escape key on your keyboard.
4.) Then type in "event" and paste the id you copied earlier. For this example, it would be "event season_events.099". And hit enter.
5.) Now you're all set! It should trigger the season that is historically accurate, however the seasonal shift cycle may be off by up to 11 months, unless the start date you have is the same as the season shift event date. You can find a different event, if there is one in a different year that's closer, and use that ID instead. Alternatively you could :

6.)Use the console command "event ALT_ID", which the ALT_ID being the event id that comes after trigger_event, just above the original ID you copied, in the code. This will set the season to the one before the one that your chosen Event Will.
7.) Use the command(3) "effect = { trigger_event = { id = YOUR_EVENT_ID days = DAYS } }
a.) For finding the days, calculate the difference between the event you wish to trigger, and your start date. Note that if your start date comes after the season shift, switch the event ID with the next one in the sequence. I recommend using : https://www.timeanddate.com/date/duration.html for calculating the difference between dates easily.
8.) And now it should work if all was inputted correctly.


This method is recommended if you want to make a compatch for yourself, or for your submod. If you intend to make a compatch using this method, let me know and I'll add a link in the description.

1.) You have two options with this method, you can either directly edit the Seasons Mod in your files, or you can make a new mod. However, your changes will be reverted whenever I update the main mod, but if you make a new mod, you'll need to update it every time there is a new update if you want the rest of the mod to work correctly.
For those with less modding experience, I'd recommend just editing the seasons file on your computer, and then saving the changes you make to a textfile elsewhere for easy reference.
2.) Begin by opening the season_events file in the events folder of the mod. There is a custom start date template provided, change the dates and follow the instructions in there. Look for : "###CUSTOM START DATE EDIT ME!###"
3.) Save the file, and launch the game, and all should work correctly!


Footnotes :
1: If your year isn't there, try searching the years around it, it shouldn't be more than 5 years earlier or later. If there is a large gap, I'd recommend trying the modding method.
2: Go to game launch options, and type in -debug_mode on steam, or go to game options in the CK3 Launcher, and launch in debug mode.
3: For this, you'll want to choose the the previous season ID