Index: src/wp-admin/options.php
===================================================================
--- src/wp-admin/options.php	(revision 37858)
+++ src/wp-admin/options.php	(working copy)
@@ -168,10 +168,18 @@
 
 	if ( 'general' == $option_page ) {
 		// Handle custom date/time formats.
-		if ( !empty($_POST['date_format']) && isset($_POST['date_format_custom']) && '\c\u\s\t\o\m' == wp_unslash( $_POST['date_format'] ) )
+		$using_locales_default_date_format = false;
+		$using_locales_default_time_format = false;
+		if ( !empty($_POST['date_format']) && isset($_POST['date_format_custom']) && '\c\u\s\t\o\m' == wp_unslash( $_POST['date_format'] ) ) {
 			$_POST['date_format'] = $_POST['date_format_custom'];
-		if ( !empty($_POST['time_format']) && isset($_POST['time_format_custom']) && '\c\u\s\t\o\m' == wp_unslash( $_POST['time_format'] ) )
+		} elseif ( $_POST['date_format'] === __( 'F j, Y' ) ) {
+			$using_locales_default_date_format = true;
+		}
+		if ( !empty($_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'];
+		} elseif ( $_POST['time_format'] === __( 'g:i a' ) ) {
+			$using_locales_default_time_format = true;
+		}
 		// Map UTC+- timezones to gmt_offsets and set timezone_string to empty.
 		if ( !empty($_POST['timezone_string']) && preg_match('/^UTC[+-]/', $_POST['timezone_string']) ) {
 			$_POST['gmt_offset'] = $_POST['timezone_string'];
@@ -222,6 +230,14 @@
 		} else {
 			unload_textdomain( 'default' );
 		}
+
+		// Set date/time formats to defaults if they were used before the update.
+		if ( $using_locales_default_date_format ) {
+			update_option( 'date_format', __( 'F j, Y' ) );
+		}
+		if ( $using_locales_default_time_format ) {
+			update_option( 'time_format', __( 'g:i a' ) );
+		}
 	}
 
 	/**