function esDigito(sChr){
  var sCod = sChr.charCodeAt(0);
  return ((sCod > 47) && (sCod < 58));
}

function valSep(oTxt){
  var bOk = false;
  bOk = bOk || ((oTxt.value.charAt(2) == "-") && (oTxt.value.charAt(5) == "-"));
  bOk = bOk || ((oTxt.value.charAt(2) == "/") && (oTxt.value.charAt(5) == "/"));
  return bOk;
}

function finMes(oTxt){
  var nMes = parseInt(oTxt.value.substr(3, 2), 10);
  var nRes = 0;
  switch (nMes){
    case 1: nRes = 31; break;
    case 2: nRes = 29; break;
    case 3: nRes = 31; break;
    case 4: nRes = 30; break;
    case 5: nRes = 31; break;
    case 6: nRes = 30; break;
    case 7: nRes = 31; break;
    case 8: nRes = 31; break;
    case 9: nRes = 30; break;
    case 10: nRes = 31; break;
    case 11: nRes = 30; break;
    case 12: nRes = 31; break;
  }
  return nRes;
}

function valDia(oTxt){
  var bOk = false;
  var nDia = parseInt(oTxt.value.substr(0, 2), 10);
  bOk = bOk || ((nDia >= 1) && (nDia <= finMes(oTxt)));
  return bOk;
}

function valMes(oTxt){
  var bOk = false;
  var nMes = parseInt(oTxt.value.substr(3, 2), 10);
  bOk = bOk || ((nMes >= 1) && (nMes <= 12));
  return bOk;
}

function valAno(oTxt){
  var bOk = true;
  var nAno = oTxt.value.substr(6);
  bOk = bOk && ((nAno.length == 2) || (nAno.length == 4));
  if (bOk){
    for (var i = 0; i < nAno.length; i++){
      bOk = bOk && esDigito(nAno.charAt(i));
    }
  }
  return bOk;
}

function valFecha(oTxt){
  var bOk = true;
  if (oTxt.value != ""){
    bOk = bOk && (valAno(oTxt));
    bOk = bOk && (valMes(oTxt));
    bOk = bOk && (valDia(oTxt));
    bOk = bOk && (valSep(oTxt));
    if (!bOk){
      // alert("La data no es correcte.");
      oTxt.value = "";
      oTxt.focus();
    }
  }
  return(bOk);
}