The web map time slider in Portal for ArcGIS displays incorrect start time between the feature layer and the map image layer depending on the specified system time zone.
This defect was logged against a version of the software which is no longer supported. The issue is addressed in ArcGIS Enterprise Map Viewer (4.x JS SDK) at ArcGIS Enterprise 11.3 with the new webmap time zone settings. The Map Viewer Classic (3.x JS SDK) is now deprecated (Refer this deprecation notice link: https://support.esri.com/en-us/knowledge-base/deprecation-map-viewer-classic-deprecation-000030662). It is recommended that users use the Map Viewer for all their workflows