Table: buffalo_bills_1980_games_2
Columns: game
date
opponent
result
bills_points
opponents
bills_first_downs
record
attendance

Question: Which date did the oakland raiders play as the opponent when the Bills first downs were under 28?
SQL: SELECT "date" FROM buffalo_bills_1980_games_2 WHERE "bills_first_downs" < 28 AND "opponent" = 'oakland raiders'