Has anyone ever seen UTC timestamps of the format xxx:HH:mm:ss.sss? What are the three x's at the front. For reference, this timestamp belongs to an upcoming launch in June of 2019.
To confirm other answers that the leading three-digit number is day number for the year starting with 1 being 01-Jan:
Note the counter in the lower-left of the video [ISS] Launch of Antares Rocket with Cygnus CRS-2 Spacecraft to ISS is
194 16:52:14.xxx at launch, corresponding to a 13 July 2014, 16:52:14 launch time, with 13-July-2014 being the 194th day of 2014
If you are seeing a three digit
006 immediately before hours for a launch in June, then something is strange.