Better date handling

[cmutel]

The current handling of start and end dates was... very creative. This is better:

• It doesn't assume that endDate is present just because startDate is.
• It can handle inputs and outputs of dates >= day 10 in the month
• It has tests for most (all?) possible combinations
• It uses builtin functionality for date handling
• It doesn't default to January 1st for end dates, but correctly finds the last day and month as needed
• It checks that start < end
• It puts tests fixtures in the tests directory

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.92%. Comparing base (5182564) to head (7d9be29).
Report is 8 commits behind head on main.

❗ Current head 7d9be29 differs from pull request most recent head ec1bb79. Consider uploading reports for the commit ec1bb79 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #49      +/-   ##
==========================================
+ Coverage   99.53%   99.92%   +0.38%     
==========================================
  Files           7        7              
  Lines        1280     1269      -11     
==========================================
- Hits         1274     1268       -6     
+ Misses          6        1       -5     

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[cmutel]

The readthedocs error doesn't have anything to do with this PR, and it looks like the docs build has been broken for a while.