Codechange: Use proper date types in various places

[2TallTyler]

Motivation / Problem / Description

Code picked from #10761 that needs to be changed anyway.

Limitations

int mo in graph_gui.cpp, just above a change I made to yr is weird and probably shouldn't be a TimerGameCalendar::Month, so I left that alone (same as in #10761).

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker