wordpress/wp-admin/js/comment.js

addLoadEvent( function() {
	add_postbox_toggles('comment');

	jQuery('#timestampdiv').css('display', 'none');
	jQuery('.edit-timestamp').click(function () {
		if (jQuery('#timestampdiv').is(":hidden")) {
			jQuery('#timestampdiv').slideDown("normal");
		} else {
			jQuery('#timestampdiv').hide();
		}
    });
});