Extracting Real-Time Start Timestamp from Flight Controller Logs for GPS Sync

I am trying to get Ist or ust time from the flight controller in bin file so that I could get date time and accordingly the gps data. I am now getting data in milliseconds which I believe is the starting time or the counter time of the controller.

timestamp(ms) ATT.Roll ATT.Pitch
611986.596 1.21 -1.88
612186.596 1.22 -1.89
612386.596 1.22 -1.88
612586.596 1.21 -1.88
612786.596 1.2 -1.88
612986.596 1.2 -1.87
613186.596 1.2 -1.88