YAHOO.namespace("my.optselect");

function updateTime() {
  var tourSelect = document.getElementById('field_id_37');
  var timeSelect = document.getElementById('field_id_38');

  if (tourSelect.options[0].selected) {timeSelect.disabled=true;} else {timeSelect.disabled=false;}
}

function init() {
  var timeSelect = document.getElementById('field_id_38');
  timeSelect.disabled=true;

  YAHOO.util.Event.addListener("field_id_37", "change", updateTime);
}

YAHOO.util.Event.addListener(window, "load", init);