time difference in the (consecutive Rows) timeStamp and TimeInterValSince1970 is wrong

Hi, there i really love your work but i am facing problem when i try to get difference of two time in the consecutive rows let say we set the frequency to 60 that means we will get 60 values in one seconds and each time we are getting time and saving it to dataBase (core data) if we are getting the value at regular interval the we should get a constant value by taking differece between two consecutive row of the time stamp for ref i am attaching a screen shot can any one tell me what is the problem in the calculation ?

Hi. These are basically the same dates but in different formats. And it seems different timezones. There must be a bug there.

any suggestion