In Space Seed, it was mention had been fought in the 1990s. But in the same episode and later in ST'TWOK, Both Spock and later in Khan stated that it was fought 200 years ago of present date. With would put it being fought sometime in the 2060s-2080s.
Like I said, if they give a specific year - like they did in TWOK - that takes precedence. Any time they say things like "200 years ago", that can be dismissed as a rounding-off.
Besides, TWOK isn't even consistent with its own self. The opening text says "In the 23rd century...", yet Khan later gives both the date of 1996 *and* mentions "200 years ago."