Hi Ajit,
This is known as a “Unix Timestamp” (number of milliseconds since January 1st, 1970, 00:00:00 GMT). In Apex Code, you can use DateTime.newInstance(unixTimestamp) to return a DateTime that contains the formatted date/time value. In JavaScript, use new Date(unitTimestamp) to get the same result. To convert back, in Apex Code, use myDate.getTime(), and in JavaScript, use the same function: myDate.getTime().