CREATE TABLE table_22917 ("Version" TEXT, "Code name" TEXT, "Release date" TEXT, "Desktops" TEXT, "Servers" TEXT, "Kernel Version" TEXT)

When was the release date of the operating system with kernel version 2.6.31?
SELECT "Release date" FROM table_22917 WHERE "Kernel Version" = '2.6.31'
SELECT "Release date" FROM "table_22917" WHERE "Kernel Version" = '2.6.31'