When did the Theme of bobsleigh release?
CREATE TABLE "1992_winter_olympics" (
  "date_of_issue" text,
  "theme" text,
  "denomination" text,
  "printer" text,
  "quantity" text,
  "design" text,
  "perforation" text,
  "first_day_cover_cancellation" text
);
SELECT "date_of_issue" FROM "1992_winter_olympics" WHERE "theme"='bobsleigh';