As a graduate student working for LIGO (the Laser Interferometer
Gravitational wave Observatory), one of my responsibilities was
maintaining date-time code, including accounting for leap
seconds, and calculating dates and time thousands of years in
the past. What is a leap second? Well, the Earth's rotation rate
is slowing down. So, as time goes on the length of the day is
longer. This rate is very slow. Every few years, the added
length must be taken into account, much like the leap day takes
into account that the orbital period of the Earth around the Sun
is not exactly 365 days. When a leap second must be inserted is
not predictable. The International Earth Rotation and Reference
Systems Service is tasked with making observations of the Earth,
and producing a twice-yearly report on whether a leap second is
due to be added. This comes out by email, and on their website.
The report gives a 6-month or so lead time before the leap
second is inserted, at midnight on Jan 1 and on Jul 1. So,
apparently, a leap se