diff --git a/wp-includes/customize/class-wp-customize-date-time-control.php b/wp-includes/customize/class-wp-customize-date-time-control.php
index c163ba1408..fdaa6837dc 100644
--- a/wp-includes/customize/class-wp-customize-date-time-control.php
+++ b/wp-includes/customize/class-wp-customize-date-time-control.php
@@ -100,14 +100,13 @@ class WP_Customize_Date_Time_Control extends WP_Customize_Control {
 		$timezone_info = $this->get_timezone_info();
 
 		$date_format = get_option( 'date_format' );
-		foreach ( array( 'Y', 'y', 'o' ) as $year_token ) {
-			$date_format = preg_replace( '/(?<!\\\\)' . $year_token . '/', '%1$s', $date_format );
-		}
-		foreach ( array( 'F', 'm', 'M', 'n' ) as $month_token ) {
-			$date_format = preg_replace( '/(?<!\\\\)' . $month_token . '/', '%2$s', $date_format );
-		}
-		foreach ( array( 'j', 'd' ) as $day_token ) {
-			$date_format = preg_replace( '/(?<!\\\\)' . $day_token . '/', '%3$s', $date_format );
+		$date_format = preg_replace( '/(?<!\\\\)[Yyo]/', '%1$s', $date_format );
+		$date_format = preg_replace( '/(?<!\\\\)[FmMn]/', '%2$s', $date_format );
+		$date_format = preg_replace( '/(?<!\\\\)[jd]/', '%3$s', $date_format );
+
+		// Fallback to ISO date format if year, month, or day are missing from the date format.
+		if ( 1 !== substr_count( $date_format, '%1$s' ) || 1 !== substr_count( $date_format, '%2$s' ) || 1 !== substr_count( $date_format, '%3$s' ) ) {
+			$date_format = '%1$s-%2$s-%3$s';
 		}
 		?>
 
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 763e9a9635..20d690aae4 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-RC1-42116';
+$wp_version = '4.9-RC1-42117';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.