fix: properly floor the partial results in `DateDayDifference`

[vil02]

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Describe your change:

Currently DateDayDifference('29/02/2024', '04/03/2024') gives 5. This PR fixes that. Further more:

• the errors now are thrown,

• the tests are reorganized; edited test cases were verified with the page linked in the docs,

• some minor refactoring was done.

• [ ] Add an algorithm?

• [x] Fix a bug or typo in an existing algorithm?

• [ ] Documentation change?

Checklist:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [x] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [x] All new JavaScript files are placed inside an existing directory.
• [x] All filenames should use the UpperCamelCase (PascalCase) style. There should be no spaces in filenames. Example:UserProfile.js is allowed but userprofile.js,Userprofile.js,user-Profile.js,userProfile.js are not
• [x] All new algorithms have a URL in their comments that points to Wikipedia or another similar explanation.
• [x] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.91%. Comparing base (83b4dd8) to head (3368c0d).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1629 +/- ## ========================================== + Coverage 83.89% 83.91% +0.01% ========================================== Files 377 377 Lines 19736 19733 -3 Branches 2916 2919 +3 ========================================== + Hits 16558 16559 +1 + Misses 3178 3174 -4 ```

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[vil02]

CheckDayAndMonth should ideally compute how many days a month has (based on the year) and check against that (you don't need to do this though).

A function like ParseSimpleDate could be used both in DateToDay and DateDayDifference (i.e. the input would be a string "24/03/2024" and the output would be some kind of tuple with day, month and year). It should also check if the resulting date is correct (including correct number of days for given month). I was planning to spend some time on it after this and #1628 are merged.