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