Index: src/wp-admin/includes/class-wp-site-health.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- src/wp-admin/includes/class-wp-site-health.php	(date 1573725321000)
+++ src/wp-admin/includes/class-wp-site-health.php	(date 1574087536154)
@@ -1828,6 +1828,46 @@
 		return $result;
 	}
 
+	/**
+	 * Test if the default PHP timezone is set to UTC.
+	 *
+	 * @return array The test results.
+	 * @since 5.3.1
+	 */
+	public function get_test_php_default_timezone() {
+
+		if ( 'UTC' === date_default_timezone_get() ) {
+
+			return array(
+				'label'       => __( 'Default PHP timezone setting is valid' ),
+				'status'      => 'good',
+				'badge'       => array(
+					'label' => __( 'Performance' ),
+					'color' => 'blue',
+				),
+				'description' => sprintf(
+					'<p>%s</p>',
+					__( 'Default PHP timezone setting was configured by WordPress during load. This is necessary for correct calculations of dates and times.' )
+				),
+				'test'        => 'php_default_timezone',
+			);
+		}
+
+		return array(
+			'label'       => __( 'Default PHP timezone setting is invalid' ),
+			'status'      => 'critical',
+			'badge'       => array(
+				'label' => __( 'Performance' ),
+				'color' => 'red',
+			),
+			'description' => sprintf(
+				'<p>%s</p>',
+				__( 'Default PHP timezone setting was changed after WordPress load by a <code>date_default_timezone_set()</code> function call. This interferes with correct calculations of dates and times.' )
+			),
+			'test'        => 'php_default_timezone',
+		);
+	}
+
 	/**
 	 * Return a set of tests that belong to the site status page.
 	 *
@@ -1890,6 +1930,10 @@
 					'label' => __( 'Debugging enabled' ),
 					'test'  => 'is_in_debug_mode',
 				),
+				'php_default_timezone'     => array(
+					'label' => __( 'PHP Default Timezone' ),
+					'test'  => 'php_default_timezone',
+				),
 			),
 			'async'  => array(
 				'dotorg_communication' => array(