Index: src/wp-admin/options.php
===================================================================
--- src/wp-admin/options.php	(revision 53038)
+++ src/wp-admin/options.php	(working copy)
@@ -265,13 +265,13 @@
 		if ( ! empty( $_POST['date_format'] ) && isset( $_POST['date_format_custom'] )
 			&& '\c\u\s\t\o\m' === wp_unslash( $_POST['date_format'] )
 		) {
-			$_POST['date_format'] = $_POST['date_format_custom'];
+			$_POST['date_format'] = !empty( $_POST['date_format_custom'] ) ? $_POST['date_format_custom'] : get_option('date_format');
 		}
 
 		if ( ! empty( $_POST['time_format'] ) && isset( $_POST['time_format_custom'] )
 			&& '\c\u\s\t\o\m' === wp_unslash( $_POST['time_format'] )
 		) {
-			$_POST['time_format'] = $_POST['time_format_custom'];
+			$_POST['time_format'] = !empty( $_POST['time_format_custom'] ) ? $_POST['time_format_custom'] : get_option('time_format');
 		}
 
 		// Map UTC+- timezones to gmt_offsets and set timezone_string to empty.