CREATE TABLE table_31921 ("Outcome" TEXT, "Year" FLOAT, "Championship" TEXT, "Opponent in the final" TEXT, "Score  ( l )  = score in legs ,   ( s )  = score in sets." TEXT)

Tell me the sum of Year for outcome of runner-up for world darts championship
SELECT SUM("Year") FROM table_31921 WHERE "Outcome" = 'runner-up' AND "Championship" = 'world darts championship'
SELECT SUM("Year") FROM "table_31921" WHERE "Championship" = 'world darts championship' AND "Outcome" = 'runner-up'