New to fitbit

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Hello all!

I bought the FitBit Zip and I wanted to know if it is fairly accurate? I plan on wearing it all day, and I usually go to the gym daily. I noticed that my calories burned on fitbit are higher than what information I plug in on the treadmill. Also, today I looked at my exercise after logging my food for the day and it said that my FitBit count was negative. What does that mean?

Thank you!!