Configure global fallbackFn for timezone-mock to handle Date subclasses

[vishnupprajapat]

What?

Part of #78005

This PR configures a global fallbackFn for timezone-mock in the Jest unit test setup. It allows the MockDate constructor to correctly handle objects that implement valueOf(), specifically addressing compatibility issues with Date subclasses like UTCDateMini.

Why?

The upgrade to date-fns v4 introduces logic that reconstructs dates using new date.constructor(date). When date-fns v4 is used alongside timezone-mock, this triggers a "Unhandled type passed to MockDate constructor: object" error because the mock does not natively support object arguments.

By setting a fallbackFn globally, we ensure that these objects are safely converted back to timestamps before being passed to the mock constructor, unblocking the monorepo-wide upgrade to date-fns v4.

How?

A global configuration has been added to test/unit/config/global-mocks.js. This file is loaded by Jest before every test suite.
Implementation:

timezoneMock.options( {
	fallbackFn: ( p ) =>
		new timezoneMock._Date(
			typeof p?.valueOf === 'function' ? p.valueOf() : p
		),
} );

This intercepts any "unhandled" types in the MockDate constructor and uses the underlying _Date (native Date) to handle the value.

Testing Instructions

1. Run the existing unit tests for date-time components:
   npm run test:unit -- packages/components/src/date-time
2. Verify that all 118 tests pass successfully.
3. This change is intended to be transparent to existing date-fns v3 tests while providing the necessary infrastructure for v4 compatibility.

Testing Instructions for Keyboard

N/A - This is a change to the test infrastructure and does not affect the user interface.

Screenshots or screencast

N/A

Use of AI Tools

This pull request was authored with the assistance of Claude Opus 4.7. The AI was used to investigate the root cause of the timezone-mock constructor errors, propose the fallbackFn solution based on maintainer suggestions, and verify the implementation by running the affected test suites.

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Co-authored-by: vishnupprajapat <vishnuprajapat@git.wordpress.org>
Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: manzoorwanijk <manzoorwanijk@git.wordpress.org>

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[Copilot]

Pull request overview

Configures timezone-mock globally in the Jest unit-test setup to support Date subclasses (e.g. UTCDateMini) by providing a fallbackFn that converts object inputs via valueOf() before delegating to the underlying native Date constructor. This is intended to prevent date-fns v4’s new date.constructor(date) reconstruction path from failing under the mocked Date.

Changes:

• Add a global timezone-mock fallbackFn in the Jest unit test setup (global-mocks.js) to handle object inputs (notably Date subclasses) by converting them to timestamps.

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[ciampo]

LGTM 🚀