Time measurement not affected by gravity?

I'm watching Star Trek, and one of the characters used hours as a unit of time. In many sci-fi series, including Trek, they use "Earth years" as another unit, which uses Earth as a standard for time. In a galactic civilization -- or really any multi-planetary civilization -- this would be relatively inefficient, basing time on a constant of a single planet. It would be more efficient to base it on something that is constant throughout the universe. Because gravity and relative velocity affect time, there aren't a ton of options here. What would be a good standard for time that could be measured anywhere in the universe, be it on a small asteroid or a massive star?