Table:
text: Chiefs–Raiders rivalry
1990s (Chiefs 18-3)
1990s (Chiefs 18-3)
Year_ Date_ Winner Result_ Loser Location_
1997 December 7 Kansas City Chiefs 30–0 Oakland Raiders Arrowhead Stadium
1998 September 6 Kansas City Chiefs 28–8 Oakland Raiders Arrowhead Stadium
1998 December 26 Kansas City Chiefs 31–24 Oakland Raiders Oakland-Alameda County Coliseum
1999 November 28 Kansas City Chiefs 37–34 Oakland Raiders Network Associates Coliseum
1999 January 2 (2000) Oakland Raiders 41–38 (OT) Kansas City Chiefs Arrowhead Stadium

Question: What is the average year of games between Chiefs-Raiders rivalry in 1990s (Chiefs 18-3) dated December 26?