Why aren't my gamerules being saved?
Yes, I know it's a bit outdated but have you tried editing the level.dat?
Just download this program and run it. Open the menu File > Open and then select the level.dat in C:/Users/[me]/AppData/Roaming/.minecraft/[matching mc world]/level.dat. Then unfold to the followings Data > GameRules > doDaylightCycle. Double-click to edit it and enter false. Save and start the game.