They arrived at that date because that's when Voyager recorded a permanent drop in heliosphere - produced particles and an increase in galactic cosmic rays from outside the solar system. (latimes.com)
2) Solar heliosphere modulation of Galactic Cosmic Rays. (realclimate.org)
The Voyager 1 launched in 1977 (along with the Voyager 2 sixteen days later) has left heliosphere and is now zooming into interstellar space. (educationworld.com)