It looks like a Javascript timestamp, the number of milliseconds since the beginning of the epoch. Dividing by 1000 should give you the UNIX timestamp. See PHP code below.
function js_to_unix_timestamp($jsTimestamp){
return $jsTimestamp/1000;
}
$timestamp = 1396017000991; //Expect 3/28/14 7:30am
echo date('r', js_to_unix_timestamp($timestamp));
//Actual Fri, 28 Mar 2014 15:30:00 +0100
//Correct to within some UTC offset
$timestamp = 1396018800571; //Given in question
echo date('r', js_to_unix_timestamp($timestamp));
//Actual Fri, 28 Mar 2014 16:00:00 +0100
//Correct?
$timestamp = 1396032315623; //Current time given by Date.now() in javascript
echo date('r', js_to_unix_timestamp($timestamp));
//Actual Fri, 28 Mar 2014 19:45:15 +0100
//Correct