How to Sort Custom columns in admin

I have set a custom column for date in admin, but the sorting is not working, when I click to sort the dates are jumbled up and not sorted correctly. This is the sorting function Im using

function sortable_columns() {
    return array(
        // meta column id => sortby value used in query
        'publishdate' => 'date', // e.g. 1344964575            
    );
} add_filter( "manage_edit-post_sortable_columns", "sortable_columns" );