Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_31921 ( "Outcome" text, "Year" real, "Championship" text, "Opponent in the final" text, "Score (l) = score in legs, (s) = score in sets." text )

### Question ###
Tell me the sum of Year for outcome of runner-up for world darts championship

### Accurate SQL ###

SELECT SUM("Year") FROM table_31921 WHERE "Outcome" = 'runner-up' AND "Championship" = 'world darts championship'