For Community Challenges, we use a user’s local time zone to determine daily steps. As a result, you may see your competitor accumulate steps before your day starts or after it has ended. Check your competitor’s local time zone on their profile to determine if their day starts before or after yours.
For Private Challenges that users create themselves we use the creator's time zone to determine the start and end times. All challenges begin at midnight of the start date and end at 11:59 pm of the end date.