Fix and enhance core time-coordinate rule

[forman]

Closes #33

Checklist (strike out non-applicable):

• Changes documented in CHANGES.md
• Related issue exists and is referred to in the PR description and CHANGES.md
• Added docstrings and API docs for any new/modified user-facing classes and functions
• Changes/features documented in docs/*
• Unit-tests adapted/added for changes/features
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.52%. Comparing base (c247a7e) to head (13f7332).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
+ Coverage   97.47%   97.52%   +0.04%     
==========================================
  Files          62       62              
  Lines        2459     2503      +44     
  Branches      346      357      +11     
==========================================
+ Hits         2397     2441      +44     
  Misses         31       31              
  Partials       31       31              

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