Firebase TIMESTAMP to date and Time

Firebase.ServerValue.TIMESTAMP is not actual timestamp it is constant that will be replaced with actual value in server if you have it set into some variable.

mySessionRef.update({ startedAt: Firebase.ServerValue.TIMESTAMP });
mySessionRef.on('value', function(snapshot){ console.log(snapshot.val()) })
//{startedAt: 1452508763895}

if you want to get server time then you can use following code

  fb.ref("/.info/serverTimeOffset").on('value', function(offset) {
    var offsetVal = offset.val() || 0;
    var serverTime = Date.now() + offsetVal;
  });

timestamp is an object

timestamp= {nanoseconds: 0,
seconds: 1562524200}

console.log(new Date(timestamp.seconds*1000))

In fact, it only work to me when used

firebase.database.ServerValue.TIMESTAMP

With one 'database' more on namespace.