Challenges start at the moment that is midnight for the person who started the challenge.
It's not just the time difference that matters but which direction. If your friends' midnight is 11:00 PM for you, then it will be 1 more hour until you hit midnight again. If their midnight is 1:00 AM for you, then it will be 23 more hours until you hit midnight again.
Best AnswerI have the same issue. I am in the Eastern Time Zone in the USA and accept challenges from someone in Switzerland. Fitbit reports an 18 hour time difference. Once in the middle of a challenge it switched from 17 hours to 18 hours and suddenly there was an extra hour for the challenge.
Best Answer