The Get a meeting activities report endpoint returns activity_time as a string with no timezone/offset (see the activity_time example in the official ZoomMeetingAPI-spec.json). Since it’s not an ISO-8601 date-time in the spec, you can’t infer the timezone from the value alone.
Zoom staff have confirmed that meeting Dashboard/Reports timestamps are returned in GMT (UTC), so you should interpret activity_time as UTC and convert for display as needed (forum confirmation). Keep the stored value in UTC and apply the viewer’s timezone when rendering.
If you need the meeting’s configured timezone, fetch it via List meetings (the response includes a timezone field per ZoomMeetingAPI-spec.json) and apply that zone when converting activity_time. This keeps output consistent for hosts across multiple timezones