Here is the spec for the logfiles that we write for a collection of impressions.

                number of impressions               : 1 byte
                milliseconds since start of the day : 4 bytes
                original query time                 : 8 bytes
                List of n=length of
                  n=binder name length     : 1 byte
                  binder name              : n bytes
                  promo ID                 : 8 bytes
                  impression time          : 8 bytes