Get Total Seconds In javascript

I am using Eonasdan datetimepicker. In formatting date it contains PM and AM and so I love to use it for my date in order to identify time easily. But when I am creating some condition via Javascript to calculate the total seconds of two given times with AM and PM text it alerts Nan. What is the best solution to get the total number of seconds with AM and PM text given?

The codes are the following:

<script type="text/javascript">
    var datetime_in= '06/30/2017 7:56 AM';
    var datetime_out= '06/30/2017 5:16 PM';

    var totalseconds= datetime_in - datetime_out;
    alert(totalseconds);
</script>