Note: this post is unfinished, but put up because I was using it to procrastinate over writing up other things.
In a recent post, I covered clocks and time systems, and presented an alternative, more regular method of telling time within the day. A natural extension of this topic is telling time beyond the day: the calendar, with its multitudinous and inconsistent larger units of time: the week (seven days), the month (28 to 31 days), the seasons (roughly a quarter of the year) and the year itself (365 to 366 days). However, before getting onto this topic, it is first necessary to discuss what happens when different unit systems collide.
In ancient times, there were all sorts of astrological events that held significance for the general population. The day dictates when they wake and sleep, the month dictates the tides and planting schedules, and the seasons, which marked the year, indicated whether they should leave the hay out to dry or stable the cows. The Earth, the Moon, and the Sun were all important parts of their daily lives. With this in mind, it is not surprising that they kept track of them all with care.
The next thing they did was to apply that marvellous human tendency to try to make things fit together: they attempted to unify the units. They didn't combine very well, and sometimes they varied, but they lacked the measuring techniques to discover that, so they (eventually) chose close approximations: about thirty days to the month, and twelve months to the year. This made calculations a little awkward, but it wasn't too bad: they are nice whole numbers, and fairly round ones at that.
Today, however, we find ourselves increasingly preoccupied with accuracy. The ability to measure more precisely has imparted to us the desire to do so with everything, and it has quickly become apparent that the old, neat ways of doing things are simply not precise enough. There are not thirty days to the month, nor twenty-four hours to the day, nor twelve (lunar) months to the year. In our quest for ever-increasing accuracy, we find we are making ever more complicated systems. The system of UTC and leap-seconds is such that it is not possible to accurately calculate a time that is six months in the future, because it is as likely as not that a leap second would have occurred.