Index: src/wp-includes/customize/class-wp-customize-date-time-control.php
===================================================================
--- src/wp-includes/customize/class-wp-customize-date-time-control.php	(revision 42113)
+++ src/wp-includes/customize/class-wp-customize-date-time-control.php	(working copy)
@@ -109,6 +109,12 @@
 		foreach ( array( 'j', 'd' ) as $day_token ) {
 			$date_format = preg_replace( '/(?<!\\\\)' . $day_token . '/', '%3$s', $date_format );
 		}
+		foreach ( array( '%1$s', '%2$s', '%3$s' ) as $required_placeholder ) {
+			if ( false === strpos( $date_format, $required_placeholder ) ) {
+				$date_format = '%2$s %3$s, %1$s';
+				break;
+			}
+		}
 		?>
 
 		<# _.defaults( data, <?php echo wp_json_encode( $data ); ?> ); #>