With a synchronous AJAX call, DOM updates made shortly before the AJAX call do not occur until the call is complete. With a series of synchronous calls, updates in any of them will not occur until series is complete. 

document.getElementById('txtboxId').select()

if(preg_match('/^(0?[1-9]|1[0-2])\/(0?[1-9]|[12][0-9]|3[01])\/((19|20)\d{2})$/', $_POST['start_date']) == 0)
echo 'The given date is not in the format(mm/dd/yyyy)

UNIX_TIMESTAMP(STR_TO_DATE( 'mm/dd/yyyy', '%m/%d/%Y' ))

var timeStamp = Date.UTC('year','month','date', 'hour','min','seconds')/1000;
var timeStamp = Date.parse(dateObj/1000);