[etaoins]

There is Barycentric Coordinate Time which is the time from a hypothetical clock at rest at the centre of mass of the Solar System. This is easier to calculate across spacecraft than an Earth-centric time would be. However, because it's outside of Earth's gravity well it ticks slightly faster than Earth time.

[tim333]

If it was at rest at the centre of mass of the Solar System wouldn't it be in the sun's gravity well?

Edit - I looked it up - it's "a clock that performs exactly the same movements as the Solar system but is outside the system's gravity well."

[Retric]

While true it’s still trivial to do such calculations and then output in GMT or whatever. In theory such calculations can get really messy, but atomic clocks are still not accurate enough to make a huge range of minor issues important.

For example we are not just in the sun’s and earth’s gravity field but also the other planets as well. So, with enough precision you would need to account for their locations.