I've only traveled for work a few times over the years, but... all ttravel time is done on the clock. I once had to male a 6 hour drive on a Sunday - but I put it on my time sheet. I once flew on a Sunday - but I put it on my time sheet. I have often had to drive 2 hours to a job site, work 12 hours, and drive 2 hours home - I put 15 hours on my time sheet (subtracted an hour for lunch).

Work related travel trime, outside of the normal commute, is work time. Whether it occurs during the core hours, overnight, or on a weekend is irrelevant. Any company or supervisor who says otherwise is setting themselves up for trouble.