Codechange: Add a DateToYear function instead of dividing each time

[2TallTyler]

Motivation / Problem

We do a lot of math with dates, so #10761 needs to do a lot of casting.

Description

Instead of repeatedly dividing by DAYS_IN_LEAP_YEAR to get the year from a date, add a function that does this calculation. It's more readable and we only have to cast once.

Limitations

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