(svn r5999) -Feature: change the original date format to a 32 bits format based at the year 0.
The game date subsystem now allows someone to start in the year 0 and continue
up to the year 5 000 000. However, you currently cannot build anything before
1920 as there is no newgrf support for dates before 1920 or after 2090 yet.

(svn r5975) -Cleanup: use ORIGINAL_BASE_YEAR & ORIGINAL_MAX_YEAR where the functions really depend on the original date format.
-Cleanup: use DAYS_TILL_ORIGINAL_BASE_YEAR where the date variables are in the old date format and conversions are needed.
-Cleanup: replace one (forgotten) instance of uint16 with Date.

(svn r5918) -Cleanup: rename ConvertDayToYMD/ConvertYMDToDay as they really convert a Date to/from a YearMonthDay.
-Cleanup: swap ConvertDateToYMD's parameters to make the order consistent with the name of the function.

(svn r5916) -Cleanup: use MIN_YEAR/MAX_YEAR for the year boundaries and BASE_YEAR when comparing _cur_year with a 'full' year.
-Cleanup: replace some magic '1920' values with BASE_YEAR.

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-Cleanup: make original comments doxygen compatible and remove/change outdated comments