Carbon::setTestNow() leaks frozen clock between PHPUnit tests

To prevent the frozen clock from leaking between PHPUnit tests, you must explicitly reset the static state in the test teardown. Because Carbon::setTestNow() stores the instance in a static property, it persists for the entire duration of the PHP process unless manually cleared.

How to Reset the Clock

The safest method to ensure isolation is to call Carbon::setTestNow(null) within the tearDown() method of your base test class. This ensures that even if a test fails or completes, the clock is restored to the system default for the next test.

protected function tearDown(): void
{
    parent::tearDown();
    Carbon::setTestNow(null);
}

Handling Carbon\Immutable

Yes, CarbonImmutable shares the same global test-now state as Carbon. When you call Carbon::setTestNow(), both classes will return the frozen time because they reference the same internal static setter within the Carbon library.

Verification and Assertions

To confirm the frozen time is working and that isolation is maintained, use the following assertion strategy:

• Confirm Freeze: Assert that Carbon::now() matches your specific instance.
• Confirm Isolation: Set a frozen time in one test, then in a subsequent test assert that Carbon::now() reflects the actual system time.

public function test_frozen_time()
{
    $now = Carbon::create(2026, 11, 11, 12, 0, 0);
    Carbon::setTestNow($now);
    $this->assertEquals($now, Carbon::now());
}

public function test_isolation()
{
    // This will fail if setTestNow(null) was not called in tearDown()
    $this->assertNotEquals(2026, Carbon::now()->year);
}

Assumptions and Uncertainty

This solution assumes you are using Carbon 2.x or 3.x. While the reset behavior is consistent across these versions, the internal implementation of the static storage may vary. If you are using a parallel test runner like ParaTest, leakage will not occur between different worker processes, as each process maintains its own memory space.