Debe haber algo mal en otra parte... este es todo el código.
<script language = "javascript" type="text/javascript">
function incrementar(elem){
document.getElementById(elem).value = parseInt(document.getElementById(elem).value,10) + 1;
}
function deNuevo(){
//alert("A");
document.getElementById('tx3').value = parseInt(document.getElementById('tx1').value);
document.getElementById('tx1').value = "0";
}
</script>
</head>
<body>
<form id="form1" name="form1" method="post" action="">
<table width="614" border="1">
<tr>
<td width="106">6 am - 2:30 pm</td>
<td width="13"> </td>
<td width="87">
<label>
<input type="button" name="1" id="boton1" value="Agregar" onclick="incrementar('tx1');"/>
</label>
</td>
<td width="16"> </td>
<td width="144"><label>
<input type="text" name="tx1" id="tx1" value="0"/>
</label></td>
<td width="9"> </td>
<td width="193"><input type="text" name="tx3" id="tx3" value="0"/></td>
</tr>
<tr>
<td>6 am - 14:30 pm</td>
<td> </td>
<td><input type="button" name="2" id="boton2" value="Agregar" onclick="incrementar('tx2');"/></td>
<td> </td>
<td><input type="text" name="tx2" id="tx2" value="0"/></td>
<td> </td>
<td><input type="text" name="tx4" id="tx4" value="0"/></td>
</tr>
</table>
<p>
<input type="button" name="3" id="boton3" value="De nuevo" onclick="deNuevo();" />
</p>
<p> </p>
</form>
</body>
</html>