On Count.It, there are two time zones that matter: the time zone of the group, and that of the user.
All challenges and group leaderboards are run based on the time zone of the group: when a person (Group 'Captain/Admin') creates a group for the first time, they are asked to input a location. This can either be a US Zip Code, or a non-US city. The group is then assigned to a time zone based on that location. This is a one time assignment. Additional members are NOT asked to input a location. Any group admin, however, can change the group's official location on their Admin --> Group Profile page. Ex:
A user's Count.It logs will match up with the daily logs shown on their fitness tracker: when Count.It syncs with a user's fitness tracker it simply keeps the activity in the time zone designated by the fitness tracking app. The time zone of a fitness tracker is typically set automatically by the phone to the user's current location, and there's no reason to change it. It is not a setting that can be changed on Count.It. So, while the challenge starts and ends in the time zone of the group, a user's activity log data will remain in their own time zone.
Please note: Count.It allows for a 12 hour syncing grace period at the end of challenges. Count.It will credit any activity logged before the challenge officially starts, or after it ends, as long as that activity is assigned to a day that is within the challenge window. However, if user has activity data within the challenge window, but fails to sync within the 12 hour grace period, then they will not get credit for that activity as the challenge has officially closed.