Versions

Minecraft Version: 1.12.2
Extended Days Version: 0.2.9
Silent Lib Version: 3.0.13

Somnia: 1.9.7 / 1.9.10

I don't know how the internal workings for changing the length of minecraft days are built, but Somnia's being confused from the changed day and night length.

It's actually working fine, given that it's confused, it's just getting stuck at noon and midnight for however many ticks are added by this mod, but it's still getting stuck.
Normally, you can use a clock to sleep up to 23h59m of minecraft time, but with ED it throws you out of bed after the added day or night ticks (which ever is encountered first) have been passed.

This might be a bug that's also partially on Somnia's side, but given that the clock shown by Somnia during sleep get's stuck at noon and midnight, it seems to lack a way of being aware that the day and night length were changed.

It would be nice if you could contact the Somnia dev about this, given that there's still someone taking care of this great mod, and see what can be done about this.

And also thanks for creating the mod in the first place, without this days in minecraft feel worse than in Harvest Moon games, looked outside to check the monster situation after sleeping through the night, packing up a few things and BAM! already noon (without it).

With this mod I can actually get something done and still have daylight left <3