<script language="javascript">
var MonthData = new Array(31,0,31,30,31,30,30,31,30,31,30,31);
function change()
{
    var date =new Date();
    TextBox_StartDate.value = date.getMonth()+1 + "/1/" + date.getFullYear();
    if(month = 1)
        month = (date.getFullYear()%4==0)?29:28;
    else
        month = MonthData[month];
    TextBox_EndDate.value =date.getMonth()+1+ "/"+month+"/" + date.getFullYear();
}
</script>
<INPUT type="button" value="Button" id=button1 name=button1 onClick="change()">
<INPUT type="text" name="TextBox_StartDate" value="">
<INPUT type="text" name="TextBox_EndDate" value="">