(svn r5934) -Cleanup: forgot some conversions to Year and to Date

-Cleanup: use _cur_year instead of _date for some (year based) comparisons
-Cleanup: remove a magic number in favour of another (less) magic number

openttd.h

@@ -267,7 +267,7 @@ typedef uint AcceptedCargo[NUM_CARGO];
 typedef struct TileDesc {
 	StringID str;
 	byte owner;
-	uint16 build_date;
+	Date build_date;
 	uint32 dparam[2];
 } TileDesc;