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-Cleanup: make original comments doxygen compatible and remove/change outdated comments

date.h

@@ -0,0 +1,35 @@
+/* $Id$ */
+
+/**
+ * 1 day is 74 ticks; _date_fract used to be uint16 and incremented by 885. On
+ *                    an overflow the new day begun and 65535 / 885 = 74.
+ * 1 tick is approximately 30 ms.
+ * 1 day is thus about 2 seconds (74 * 30 = 2220) on a machine that can run OpenTTD normally
+ */
+#define DAY_TICKS 74
+
+#define MAX_YEAR_BEGIN_REAL 1920
+#define MAX_YEAR_END_REAL 2090
+#define MAX_YEAR_END 170
+
+/* Year and Date are defined elsewhere */
+typedef uint8  Month;
+typedef uint8  Day;
+typedef uint16 DateFract;
+
+typedef struct YearMonthDay {
+	Year  year;
+	Month month;
+	Day   day;
+} YearMonthDay;
+
+extern Year      _cur_year;
+extern Month     _cur_month;
+extern Date      _date;
+extern DateFract _date_fract;
+
+
+void SetDate(Date date);
+void ConvertDayToYMD(YearMonthDay *ymd, Date date);
+uint ConvertYMDToDay(Year year, Month month, Day day);
+Date ConvertIntDate(uint date);