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On your computer Dashboard, click on the gear and then settings. Scroll down to clock display time and use the drop down to select the one you want.
It must sync before it takes place
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A warm welcome to the Community @Emercl and @WendyB thanks for stopping by.
I would like to know if our friend's recommendation worked? As suggested, you can change this option only from a computer by doing the following:
1. Log into your fitbit.com dashboard and click the gear icon in the top right corner.
2. Select “Settings” from the drop-down menu.
3. Select 12-hour or 24-hour time display format here.
4. Sync your tracker to update your time zone.
Hope to hear from you soon. 
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