The Julian Calendar miscalculated the length of the solar year by 11 minutes. This concerned pope Gregory the XIII because it meant that Easter fell further away from the Spring Equinox with each passing year. The solution was the Gregorian Calendar, in which only century years that were divisible by 400 (e.g. year 2000) were leap years (instead of every century year, as in the Julian Calendar).
(https://on.natgeo.com/3iFrmst, http://bit.ly/2TlS2Tw, https://on.natgeo.com/2Ft9HGb)