If you are manually entering an activity, what you log replaces whatever Fitbit had estimated in that time. That is why you need to enter the start time and duration--so it replaces the right time period. I've accidentally entered the wrong time (usually swapping AM and PM) and I can tell you otherwise it will mess up your numbers if the activity is logged at the wrong time.
Sam | USA
Fitbit One, Macintosh, IOS
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