(svn r5975) -Cleanup: use ORIGINAL_BASE_YEAR & ORIGINAL_MAX_YEAR where the functions really depend on the original date format.

-Cleanup: use DAYS_TILL_ORIGINAL_BASE_YEAR where the date variables are in the old date format and conversions are needed.
-Cleanup: replace one (forgotten) instance of uint16 with Date.

ai/default/default.c

@@ -1457,7 +1457,7 @@ static void AiWantAircraftRoute(Player *p)
 {
 	uint16 r = (uint16)Random();
 
-	if (r >= 0x2AAA || _date < 0x3912) {
+	if (r >= 0x2AAA || _date < 0x3912 + DAYS_TILL_ORIGINAL_BASE_YEAR) {
 		AiWantPassengerAircraftRoute(p);
 	} else {
 		AiWantOilRigAircraftRoute(p);