If you are creating a module or extension with this habit, you could just include such a listbox in your code.

/Joe

function time_cells($label, $name, $value=null, $min="", $max="", $title=false,
    $labparams="", $post_label="", $inparams="")
{
      global $Ajax;

    default_focus($name);
    if ($label != null)
        label_cell($label, $labparams);
    echo "<td>";

    if ($value === null)
        $value = get_post($name);
    echo "<input $inparams type=\"time\" name=\"$name\" min=\"$min\" max=\"$max\" value=\"$value\""
        .($title ? " title='$title'" : '')
        .">";
    if ($post_label != "")
        echo " " . $post_label;

    echo "</td>\n";
    $Ajax->addUpdate($name, $name, $value);
}
function time_row($label, $name, $value, $min, $max, $title=null, $params="", $post_label="")
{
    echo "<tr><td class='label'>$label</td>";
    text_cells(null, $name, $value, $min, $max, $title, $params, $post_label);

    echo "</tr>\n";
}

What i mean is a kind of textbox wich has the format HHH:mm:ss validation.

regards

adrian