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

   function valSep(oTxt){
    var bOk = false;
    var sep1 = oTxt.value.charAt(2);
    var sep2 = oTxt.value.charAt(5);
    bOk = bOk || ((sep1 == "/") && (sep2 == "/"));
    return bOk;
   }

   function esBisiesto(nAno){
    var bRes = true;
    res = bRes && (nAno % 4 == 0);
    res = bRes && (nAno % 100 != 0);
    res = bRes || (nAno % 400 == 0);
    return bRes;
   }

   function finMes(oTxt){
    var nMes = parseInt(oTxt.value.substr(3, 2), 10);
    var nAno = parseInt(oTxt.value.substr(6), 10);
    var nRes = 0;
    switch (nMes){
     case 1: nRes = 31; break;
     case 2: nRes = 28; 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 + (((nMes == 2) && esBisiesto(nAno))? 1: 0);
   }

   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 == 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("Fecha inválida");
      oTxt.value = "";
      oTxt.focus();
     } 
    }
   }