Time Zones for Activities
When Activities are created they have a Start Date, End Date and possibly an Archive Date. If you don't choose a specific time with the date, a time is still being selected for you (i.e. 12:00AM on the start date) so all Activity dates are affected by time zones.
The time zone of an Activity will match the site's unless a Study level time zone has been set in Study Settings: Configuration.
To verify which time zone was saved with the Activity, visit the Activity Setup page for the desired Activity and press edit time below any of the date inputs as shown below. The Activity's time zone will be shown and it can be selected if you wish to modify it.