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-Cleanup: make original comments doxygen compatible and remove/change outdated comments