<input type="checkbox" id="PM" style="float: left;" onclick="ValidateCheckBox(this)" />


<script type="text/javascript">
        function ValidateCheckBox(source) {
    alert("HERE!")
    var checkbox1 = document.getElementById('AMPM');
    checkbox1.checked = false;
    var checkbox2 = document.getElementById('AM');
    checkbox2.checked = false;
    var checkbox3 = document.getElementById('PM');
    checkbox3.checked = false;
    $(source).attr('checked', true);

    var hdn1 = document.getElementById('timeAMPM');
    var hdn2 = document.getElementById('timeAM');
    var hdn3 = document.getElementById('timePM');

    hdn1.value = checkbox1.checked;
    hdn2.value = checkbox2.checked;
    hdn3.value = checkbox3.checked;
}
</script>

<script type="text/javascript">
    function ValidateCheckBox() {
        alert("HERE!")
      }
</script>

<input type="checkbox" id="PM" style="float: left;" onclick="ValidateCheckBox()" />