Thanks for me pointing me in the right direction! I forgot to put the time state in the month table.
Viewing post in World Time & Game Calendar Plugin for Godot 4 comments
I just realized the error that popped up originally was also from when I loaded the GameTimeSystem in another scene from the main scene, GameTimeSystem did not load before CalendarDdisplay tried referencing it.
I just added "await get_node("../../WorldTimeSystems/GameTimeSystem").ready" to the func refresh in CalendarDisplay