Not sure about AGOL but in Portal 10.5.1 this solution is better than enabling time properties per-layer in ArcMap because that makes a time slider appear on any web map you add the service to. We can't figure out how to make that go away, even after disabling the time slider in the map document before publishing. If you just want your time to show up correctly without having to deal with time sliders, this is best. Also, the time zone property does not exist in ArcMap 10.2.1 (wasn't introduced until 10.4 as far as I can tell), so we had to publish first then change the time zone in Server Manager.