[Happy Monkey]

Years are of varying lengths, in order to keep the average length as close as possible to an astronomical year. Centuries and decades are defined by years, not days or seconds, so they don't really need to be adjusted, as such, as long as you don't expect them to be the same number of seconds long.It's similar to the fact that a baby turning six months old will not necessarily be twice as many days old as one turning three months old, but they are twice as many months old.