Have you considered adding an option to have day extended periods increase a bit each day while night extended periods decrease the same amount? Then once the game hits a certain day, these changes would begin to reverse. This way, you can configure something like seasonal time changes without needing cross-mod integration with the Serene Seasons mod. You could also integrate other ways for extended periods to change based on certain actions, commands, or possibly with Scaling Health.

I had a neat idea for a modpack where nights start off at default length and gradually get longer every single day indefinitely.

+1 for seasonal timing!

+2 if it syncs with serene seasons

I was really looking for a mod for this, and this is the closest it would get to if this gets implemented; here are my ideas:

Using real Earth data from a 45 degree N city for an average of latitude (no weird fluctuations):
Actual time > ticks

At the solstice:
Summer:
Sunrise starts: 03:00 > 3000
Sunrise ends: 05:30 > 5500
Sunset starts: 21:00 > 21000
Sunset ends: 23:30 > 23500

Winter:
Sunrise starts: 06:00 > 6000
Sunrise ends: 07:45 > 7750
Sunset starts: 16:30 > 16500
Sunset ends: 18:20 > 18200

Fall and Spring are the same night, day, sunset, and sunrise times (default Minecraft time) at the solstice
Summer > Fall > Winter: Nights get longer, Days get shorter, pass up default times
Winter > Spring > Summer: Days get longer, Nights get shorter, pass up default times again