Source

File: wp-admin/includes/class-wp-site-health.php

public function get_test_php_default_timezone() {
		$result = array(
			'label'       => __( 'PHP default timezone is valid' ),
			'status'      => 'good',
			'badge'       => array(
				'label' => __( 'Performance' ),
				'color' => 'blue',
			),
			'description' => sprintf(
				'<p>%s</p>',
				__( 'PHP default timezone was configured by WordPress on loading. This is necessary for correct calculations of dates and times.' )
			),
			'actions'     => '',
			'test'        => 'php_default_timezone',
		);

		if ( 'UTC' !== date_default_timezone_get() ) {
			$result['status'] = 'critical';

			$result['label'] = __( 'PHP default timezone is invalid' );

			$result['description'] = sprintf(
				'<p>%s</p>',
				sprintf(
					/* translators: %s: date_default_timezone_set() */
					__( 'PHP default timezone was changed after WordPress loading by a %s function call. This interferes with correct calculations of dates and times.' ),
					'<code>date_default_timezone_set()</code>'
				)
			);
		}

		return $result;
	}