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) && (nAno % 4) == 0)? 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 == 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));  
     return bOk;  
    }  
   }  

   function fechaMayorOIgualQue(fec0, fec1){  
    var bRes = false;  
    var sDia0 = fec0.value.substr(0, 2);  
    var sMes0 = fec0.value.substr(3, 2);  
    var sAno0 = fec0.value.substr(6, 4);  
    var sDia1 = fec1.value.substr(0, 2);  
    var sMes1 = fec1.value.substr(3, 2);  
    var sAno1 = fec1.value.substr(6, 4);  
    if (sAno0 > sAno1) bRes = true;  
    else {  
     if (sAno0 == sAno1){  
      if (sMes0 > sMes1) bRes = true;  
      else {  
       if (sMes0 == sMes1)  
        if (sDia0 >= sDia1) bRes = true;  
      }  
     }  
    }  
    return bRes;  
   }  

   function valFechas(){  
    var bOk = false;  
    if (valFecha(document.f1.fec0)){  
     if (valFecha(document.f1.fec1)){  
      if (fechaMayorOIgualQue(document.f1.fec1, document.f1.fec0)){  
       bOk = true;  
       alert("Ok");  
      } else {  
       alert("Rango invalido");  
       document.f1.fec1.focus();  
      }  
     } else {  
      alert("Fecha invalida");  
      document.f1.fec1.focus();  
     }  
    } else {  
     alert("Fecha invalida");  
     document.f1.fec0.focus();  
    }  
   }  

  </script>