The documentation for the 'bbt_offset' field needs a cleanup, IMHO:

"frame offset for the BBT fields (the given bar, beat, and tick values actually refer to a time frame_offset frames before the start of the cycle), should be assumed to be 0 if JackBBTFrameOffset is not set. If JackBBTFrameOffset is set and this value is zero, the BBT time refers to the first frame of this cycle. If the value is positive, the BBT time refers to a frame that many frames before the start of the cycle. "

Since I don't know what the field does (just trying to understand the API), I'm probably not the best person to do that.