Every Count.It challenge begins at 12:00AM on its "Start" day, and it ends at 11:59:59PM on the "End" day. That said, users in different timezones will participate in the challenge in their own timezones.
Effectively, this means that the challenge starts first for users in the earliest timezone in the challenge group, and then begins in a rolling fashion as the Start day begins around the globe for other participants. Similarly, the challenge ends first for those who started first, and so on.
By letting each users compete in their own timezone, all challengers get the same amount of time in the challenge, and everyone's Count.It log will exactly reflect the activity they see in their fitness tracking app(s).
Sync Grace Period!
As noted, challenges end at midnight on the last day of the challenge, in the user's timezone. However, all challenge participants have a 12 hour "sync grace period" to make sure that data from their final challenge day gets counted.
After the grace period has ended, the challenge is officially closed, and no further activity will be accepted.
Changing timezones? Users who change timezones during a challenge will see their Count.It activities in the challenge updated to the new timezone. This means that a user may gain or lose some time and activity on one end of the challenge, but they will gain or lose the corresponding amount of time on the other side of the challenge.