Mira a ver si esto te vale
Código HTML:
...
<script>
function copia(valor){
window.day_to.value=valor;
}
</script>
<input name="day_from" onkeyup="copia(day_from.value)" type="text" class="Estilo2" id="day_from" size="10" maxlength="10">
<a href="javascript:void(0)" onClick="open_cal('day_from')"><img src="images/b_calendar.gif" width="16" height="16" border="0"></a> </td>
<td width=50>Hasta</td>