(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-Cleanup: make original comments doxygen compatible and remove/change outdated comments

ai/default/default.c

@@ -25,6 +25,7 @@
 #include "../../depot.h"
 #include "../../variables.h"
 #include "../../bridge.h"
+#include "../../date.h"
 #include "default.h"
 
 // remove some day perhaps?