fix: parse holiday date strings as local time to prevent timezone shift

[martijnrusschen]

Summary

Fixes #6105

This PR fixes a bug where holidays specified as ISO date strings (e.g., "2025-01-01") would display on the wrong day in timezones west of UTC.

The Problem

When holiday dates are provided as ISO date strings (YYYY-MM-DD), the previous implementation used new Date(string) to parse them. Per JavaScript specification, ISO date strings without a time component are parsed as UTC midnight.

Later, when the date is formatted using date-fns's format() function, it converts the date to the local timezone. This causes the date to shift backward by one day for users in timezones west of UTC.

Example of the bug:

• Input: "2025-01-01"
• Parsed as: 2025-01-01T00:00:00.000Z (UTC midnight)
• In PST (UTC-8): 2024-12-31T16:00:00.000-08:00
• Result: Holiday displays on December 31st instead of January 1st

The Fix

The fix uses the existing parseDate utility function with the "yyyy-MM-dd" format. This leverages date-fns's parse function which treats the date string as local time rather than UTC, ensuring holidays display on the intended date regardless of timezone.

Before:

const date = new Date(holiday.date);

After:

const date = parseDate(holiday.date, "yyyy-MM-dd", undefined, false);

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.23%. Comparing base (8279f2f) to head (f7c928e).
⚠️ Report is 46 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6106   +/-   ##
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  Coverage   99.23%   99.23%           
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  Files          30       30           
  Lines        3649     3649           
  Branches     1578     1574    -4     
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  Hits         3621     3621           
  Misses         27       27           
  Partials        1        1           

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