We are all accustomed to thinking of hours, days, and years as more or less immutable: There are 24 hours in a day, 365.25 days in a year, and so on – and of course that quarter of a day at the end of the year is the reason we have the Gregorian calendar, perpetual calendars, and endless proposals for calendar reform (most if not all of them as unwanted as they are impractical). However, the arrow of time points in just one direction and along the course of that arrow, all things are mutable, even the length of the day.