codice:
<script>
function su() {n=document.getElementById('numero');v=parseInt(n.value);n.value=(v +1) % 100;}
function giu() {n=document.getElementById('numero');v=parseInt(n.value);n.value=(v +99) % 100;}
</script>

<input type="button" value="<" onclick="giu()"><input type="text" id="numero" value="0" readonly><input type="button" value=">" onclick="su()">