The Portal for ArcGIS Map Viewer displays time stamp values for time enabled layers to the local time zone of the person viewing the data, regardless of time zone settings applied to the published layer. There are many applications when analysts and users need to view temporal data in the time zone it was collected. For example, it is very frustrating when the Portal map viewer converts incident data that occurred in say Paris, France to US Eastern time only because the Portal user is located in the Eastern time zone. This makes the temporal analysis and visibility map viewer tools mostly useless when viewing event data far away from one's local time zone.
Interestingly, ArcGIS Pro and ArcGIS Server seem to honor time zone settings when Portal for ArcGIS does not.