CREATE TABLE table_204_959 (id DECIMAL, "club" TEXT, "founded" DECIMAL, "nickname" TEXT, "location" TEXT, "home ground" TEXT, "entered competition" DECIMAL, "most recent promotion" DECIMAL)

what is the difference , in years , between the founding of the heidelberg football club and the eltham football club ?
SELECT ABS((SELECT "founded" FROM table_204_959 WHERE "club" = 'heidelberg football club') - (SELECT "founded" FROM table_204_959 WHERE "club" = 'eltham football club'))
SELECT ABS((SELECT "founded" FROM "table_204_959" WHERE "club" = 'heidelberg football club') - (SELECT "founded" FROM "table_204_959" WHERE "club" = 'eltham football club'))