I am using filter widget and group filter widget on feature layers with dates, and I am noticing different behaviors :
Living in Pacific, I have a timezone of UTC+11. I know now that in AGOL / Enterprise, everything is stored in UTC and then translated into local timezone by the client.
With the filter widget with the "between two dates" parameter, the filter is correctly applied, however with the group filter and the "before date" parameter, every date before 11AM is not filtered by its (local) day but by the day before.
Tested with WAB 2.15 and enterprise 10.7.1
Is this a known bug ?