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This is one of those "it depends..." questions,
You can choose between 12 and 24 hour clocks in your profile settings at https://www.fitbit.com/user/profile/edit After any changes here you need to sync to update your fitbit
However, you may have chosen a clockface that ignores this setting or has its own setting so if the setting change doesn't work check the notes for your clock in the clock gallery
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