(svn r5887) -Cleanup: move date related functions, defines and variables to date.[ch]

-Cleanup: fix whitespace related coding style issues in date.[ch]
-Cleanup: make original comments doxygen compatible and remove/change outdated comments

aircraft_cmd.c

@@ -23,6 +23,7 @@
 #include "newgrf_engine.h"
 #include "newgrf_callbacks.h"
 #include "newgrf_text.h"
+#include "date.h"
 
 static bool AirportMove(Vehicle *v, const AirportFTAClass *Airport);
 static bool AirportSetBlocks(Vehicle *v, AirportFTA *current_pos, const AirportFTAClass *Airport);