Moved date variables to dedicated singleton

Due to the growing complexity of date system, it's been moved to its own dedicated singleton.

scripts/autoload/sim_time.gd

@@ -0,0 +1,32 @@
+extends Node
+
+var year: int = 2000 setget increment_year
+var prev_year: int
+var month: int = 1 setget increment_month
+var prev_month: int 
+var day: int = 1 setget increment_day
+var prev_day: int
+
+func new_year():
+	prev_year = year
+	prev_day = day
+	prev_month = month
+	increment_year(1)
+	day = 1
+	month = 1
+	
+func reset_day():
+	prev_day = day
+	day = 1
+
+func increment_day(new_day):
+	prev_day = day
+	day += new_day
+
+func increment_year(new_year):
+	prev_year = year
+	year += new_year
+	
+func increment_month(new_month):
+	prev_month = month
+	month += new_month