Replace day length scaled tick macros with global variables.

This is to avoid needing to expensively re-calculate them all the time.
Instead update them by incrementing.

src/saveload/misc_sl.cpp

@@ -127,6 +127,7 @@ static const SaveLoadGlobVarList _date_check_desc[] = {
 static void SaveLoad_DATE()
 {
 	SlGlobList(_date_desc);
+	SetScaledTickVariables();
 }
 
 static void Check_DATE()