The topic of dates and times in web GIS is a loaded topic covered in many posts and blogs elsewhere. I just encountered it in a Web AppBuilder application we are developing. The dates in our application have no times and I configured the attributes in the layer popups to hide the times, since they should all be midnight. By chance, I forgot to configure one correctly and I saw the datetime was 7 hours behind on the previous day, meaning it was taken as UTC and adjusted to the local time zone, when it was already in the local time zone. Based on the web map JSON and research, I figured out enough that I needed to republish the map service and specify that all times are in the local time zone, probably with Daylight Saving Time adjustments.
The problem is that if I add the map service or its layers to a new web map in the Map Viewer, the dates and times display correctly, however, I already had configured a very large web map with lots of configuration and it does not automatically work. I searched the web map JSON and I see nothing different between the two web maps. There is no mention of the local timezone. There is no block of timezone configuration. I have no idea how to get my web map to pick up the timezone on the map service.