To be sure that it will work on any browser (like iOS safari) we can just split time and create new date instance.
Try:
$scope.item = {};
$scope.item.myDate = '2013-11-21T20:58:03+0000';
var arr = $scope.item.myDate.split(/[- :+T]/);
var fixedDate = new Date(arr[0], arr[1]-1, arr[2], arr[3], arr[4]);
var currTime = new Date().getTime();
var fixedTime = fixedDate.getTime();
var tot = currTime - fixedTime;
var minutes = tot / 1000 / 60;
console.log('tot', tot);
console.log('minutes', minutes);
Demo Fiddle