now = function() {
  return root.Date.now();
}
Gets the timestamp of the number of milliseconds that have elapsed since
the Unix epoch (1 January 1970 00:00:00 UTC).

@static
@memberOf _
@since 2.4.0
@category Date
@returns {number} Returns the timestamp.
@example

_.defer(function(stamp) {
  console.log(_.now() - stamp);
}, _.now());
// => Logs the number of milliseconds it took for the deferred invocation.