fix: map iso8601 to gregory calendar

[KaiVandivier]

This seems to fix an issue with the calendar that was introduced in Chrome 148, and works as I believe ISO 8601 was intended in our case

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.87%. Comparing base (d202f08) to head (d3551d5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #91   +/-   ##
=======================================
  Coverage   90.87%   90.87%           
=======================================
  Files          57       57           
  Lines        1249     1249           
  Branches      352      352           
=======================================
  Hits         1135     1135           
  Misses        112      112           
  Partials        2        2           

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud