Sorry, please help me clarify this issue.
Assume that I have information from a Landsat 8 image with the time shown as below:
Start time: 2018:288:03:13:25.1532590 Stop time: 2018:288:03:13:56.9232570
Does it mean the image was at 3:13m:25s UTC?
The format of Landsat 8 start/stop times is: YYYY:DDD:HH:MM:SS. So in your example above, the start time would be Oct. 15, 2018 at 3:13:25.1532590 (3 hours, 13 minutes, 25.1532590 seconds).
You can refer to the Landsat documentation here.