I thought it was when Walter Cronkite lied about Tet in 1968.
I asked a friend, in his nineties about the subject, thinking in the days of Edward R. Murrow the news was reported without bias.
He laughed and said it was almost as bad then.
I have a complete collection of bound Harpers magazines from the first one in the 1840s to 1910.
I stopped with 1910 as their format changed to a lot of fluff and obvious leftist bias began to appear.
The media have been biased in one manner or another since our founding, when various newspapers were quite partisan, but nowadays, of course, the bias is mainly in one direction: Communist.