The film 2001: A Space Odyssey is divided into several sections. The first, "The Dawn of Man", takes place in the distant past. This is followed by a sequence in which Heywood Floyd visits the monolith on the moon, and then after that we have the mission to Jupiter, which takes place 18 months later, according to a title card that appears on screen.
Presumably, either the second or third part of the film takes place in the year 2001. But which is it? Does Floyd visit Tycho in 1999 or so, with Bowman and co heading to Jupiter in 2001, or is the monolith discovered in 2001 with the Jupiter mission taking place a year or so later?