When was the opponent Poland and the match type EC -qualifier?
CREATE TABLE "national_team_matches" (
  "date" text,
  "location" text,
  "opponenent" text,
  "result" text,
  "match_type" text
);
SELECT "date" FROM "national_team_matches" WHERE "opponenent"='poland' AND "match_type"='ec -qualifier';