The Wrath of Khan was two-faced on the dating - the title card identified the time period as the 23rd century, yet Khan said that he ruled a large part of Earth two centuries before, which would place his rule in the late 21st century.
Maybe that was the intent. After all, the mid-1990s were a lot closer in 1982 than they were in 1967, so it's quite possible the filmmakers were retconning the timeframe of the Eugenics Wars, pushing them into the future to make it less implausible to contemporary audiences. It wouldn't have been the only retcon in TWOK -- see Khan and Chekov having a history, Khan's multiethnic followers turning into a bunch of blond Nordic types and becoming impossibly young for people who'd been stranded as adults 15 years earlier, their residences on Ceti Alpha V being adorned with movie-era equipment rather than TOS-era, Kirk having "never faced death" despite having lost Gary, Edith, Sam, and Aurelan and lived through the Tarsus IV disaster as a youth, etc. Fans today get so up-in-arms about continuity problems in new Trek productions, but they tend to overlook how cavalier TWOK, which so many of them consider the definitive Trek movie, was about continuity.