The film was marketed as a "direct sequel" to Terminator 2: Judgment Day. It doesn't literally take place the day afterwards, but the implication is that the other sequel films that were released (chronologically) after Judgement Day are being ignored, meaning that you can watch and enjoy this film as long as you've seen the first two.
“And then Tim wanted Linda,” he said. “I think the movie could have
survived having Linda in it, I think it could have survived having
Arnold in it, but when you put Linda and Arnold in it and then, you
know, she’s 60-something, he’s 70-something, all of a sudden it wasn’t
your ‘Terminator’ movie, it wasn’t even your dad’s ‘Terminator’ movie,
it was your granddad’s ‘Terminator’ movie. And we didn’t see that. We
loved it, we thought it was cool, you know, that we were making this
sort of direct sequel to a movie that came out in 1991. And young
moviegoing audiences weren’t born. They wouldn’t even have been born
for another 10 years.”
James Cameron Has Some Regrets About ‘Terminator: Dark Fate’: ‘It Was Your Granddad’s ‘Terminator’ Movie’
As to why this decision was taken, the director was pretty scathing about the quality of the previous sequel films, describing them as 'misfires'.
"The last movie was not good. Wait, I shouldn’t say that," he tells
our sister publication Total Film magazine. If Miller has a hard time
staying diplomatic about the franchise, it's only because he adores
the movies so much. “There have been some… misfires,” he laughs, “You
don’t want it to go out that way as a fan. I wanted the franchise to
have some kind of noble future again. I wanted to help with that
because I love The Terminator so, so much.”
Exclusive: Terminator: Dark Fate director on ignoring the post-Judgment Day movies: "There have been some… misfires"
