When was the XXX discovered?
Kepler-421b
Kepler-421b is an exoplanet that, as of July 2014, has the longest known year of any transiting planet (704 days), although not as long as the planets that have been directly imaged, or many of the planets found by the radial-velocity method, or as long as some transiting planet candidates which are listed as planets in the Extrasolar Planets Encyclopaedia (KIC 5010054 b etc).
2014