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$date_time_input = new XoopsFormDateTime("caption", "name_of_variable", $size_of_input_field, $timestamp_of_default_time);
That's in the form.
On the receiving end, to get a UNIX timestamp of the selected time, use this:
$timestamp = strtotime($_REQUEST['name_of_variable']['date']) + $_REQUEST['name_of_variable']['time'];