<input id="cbxBirthDate" type="checkbox" onClick="javascript:uncheckShowYear(this);" />
<input id="cbxShowYear" type="checkbox" />
<script language="javascript" type="text/javascript">
function uncheckShowYear(obj)
{
if (obj.checked == false)
{
document.getElementById("cbxShowYear").checked = false;
}
}
</script>