diff --git a/wp-includes/media.php b/wp-includes/media.php
index c7443068dd..342edd46d9 100644
--- a/wp-includes/media.php
+++ b/wp-includes/media.php
@@ -1565,8 +1565,8 @@ function wp_prepare_attachment_for_js( $attachment ) {
 		'name'        => $attachment->post_name,
 		'status'      => $attachment->post_status,
 		'uploadedTo'  => $attachment->post_parent,
-		'date'        => $attachment->post_date_gmt . ' UTC',
-		'modified'    => $attachment->post_modified_gmt . ' UTC',
+		'date'        => strtotime( $attachment->post_date_gmt ) * 1000,
+		'modified'    => strtotime( $attachment->post_modified_gmt ) * 1000,
 		'mime'        => $attachment->post_mime_type,
 		'type'        => $type,
 		'subtype'     => $subtype,