What time does a new streak day start in Eastern Time Zone?

Streaks reset at "00:00 UTC" regardless of your time zone. See here for info on UTC. So it doesn't really matter what your local time is, your streak will always reset at midnight UTC. On that note, it's worth mentioning that your activity days do not reset at "00:00 UTC", they reset at "00:00" your local time. In other words, the activity map and the streak counter are wired to reset at different times during the day, provided, of course, that you're not located in an area that uses the UTC time zone. So if your location is to the East of the UTC time zone, your activity days will reset before your streak counter because "00:00" your local time comes before "00:00 UTC". Similarly, if your location is to the West of the UTC time zone, your activity days will reset after your streak counter because "00:00" your local time comes after "00:00 UTC".

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