new file mode 100644
--- /dev/null
+++ b/src/date_type.h
@@ -0,0 +1,56 @@
+/* $Id$ */
+
+/** @file date_type.h Types related to the dates in OpenTTD. */
+
+#ifndef DATE_TYPE_H
+#define DATE_TYPE_H
+
+/**
+ * 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
+
+/*
+ * ORIGINAL_BASE_YEAR, ORIGINAL_MAX_YEAR and DAYS_TILL_ORIGINAL_BASE_YEAR are
+ * primarily used for loading newgrf and savegame data and returning some
+ * newgrf (callback) functions that were in the original (TTD) inherited
+ * format, where '_date == 0' meant that it was 1920-01-01.
+ */
+
+/** The minimum starting year/base year of the original TTD */
+#define ORIGINAL_BASE_YEAR 1920
+/** The maximum year of the original TTD */
+#define ORIGINAL_MAX_YEAR 2090
+
+/**
+ * The offset in days from the '_date == 0' till
+ * 'ConvertYMDToDate(ORIGINAL_BASE_YEAR, 0, 1)'
+ */
+#define DAYS_TILL_ORIGINAL_BASE_YEAR (365 * ORIGINAL_BASE_YEAR + ORIGINAL_BASE_YEAR / 4 - ORIGINAL_BASE_YEAR / 100 + ORIGINAL_BASE_YEAR / 400)
+
+/* The absolute minimum & maximum years in OTTD */
+#define MIN_YEAR 0
+/* MAX_YEAR, nicely rounded value of the number of years that can
+ * be encoded in a single 32 bits date, about 2^31 / 366 years. */
+#define MAX_YEAR 5000000
+
+typedef int32  Date;
+typedef uint16 DateFract;
+
+typedef int32  Year;
+typedef uint8  Month;
+typedef uint8  Day;
+
+struct YearMonthDay {
+	Year  year;
+	Month month;
+	Day   day;
+};
+
+static const Year INVALID_YEAR = -1;
+static const Date INVALID_DATE = -1;
+
+#endif /* DATE_TYPE_H */