Here is a funny thought experiment - the distance from Voyager to Earth varies by approximately 16 light minutes throughout the year. Why? Because it takes ~8 minutes for light to go from the Sun to the Earth, so presuming the Voyager is roughly planar with the Sun/Earth (I'm just assuming yes), that gives a variance of ~16 minutes depending on where the earth is on its orbit.

Now I'm presuming they aren't using the actual Earth position, but rather an average Earth position (which is basically just the Sun's position). Since Voyager is ~30 light minutes away from being 1 light-day away, that means this ~16 minute change can affect our 1 light-day mark by up to ~6 months!