Today will be a whole second longer than any other this year. This is also the longest year since 1992, the most recent previous leap year which also included a leap second. Leap seconds are used to keep standard time within 0.9 seconds of apparent solar time, due to the Earth’s changing rotational speed.
The International Earth Rotation and Reference Systems Service determines when leap seconds are needed, and announces them in bulletins such as this one. The leap seconds are applied globally at 00:00:00 UTC, so, in my case, there will be an extra second between 5 and 6 pm CST (the extra second is 17:59:60). By the way, this explains why the tm_sec field in the POSIX tm struct has the range 0-60 instead of the “expected” 0-59.
We’ve come a long way from when just glancing at the sun and guestimating the hour was sufficient for more than casual use. While you might think that knowing the time would be simple today, in fact it is an extremely complex process to determine the time. Furthermore, you have to specify which time you’re talking about. The Time Service Department of the USNO offers a brief list of time systems, such as Atomic, Universal, Coordinated Universal, Dynamical, Geocentric Coordinate, Barycentric Coordinate, and Sidereal.
The various time systems in use run at various rates, some are non-uniform, and some are even adjusted for relativistic time dilation (due to both velocity and gravity differences). Furthermore, some differ by many seconds. For example, UTC is (after today’s leap second) 34 seconds ahead of TAI, while the GPS clocks are “only” 19 seconds ahead. A down-to-earth (sorry) example of relativistic adjustments occurs in the clocks on board GPS satellites.
Fortunately, civil standard time is simply derived from UTC. Usually only specialists are concerned with the other systems — consumer products (such as GPS units or computer systems) typically display standard time, regardless of the time system used by the underlying system. Let’s not get into time zones, leap years and all the various calendar and holiday systems that abound.
It’s always good to have the correct time on your computer. However, it’s becoming a necessity because security mechanisms depend more and more on the hosts on a network (including PCs) being synchronized with each other. The Network Time Protocol has been in use for years to keep computer clocks correct and synchronized, but PCs have lagged behind — the Windows Time Service is sadly lacking, rarely achieving more than a couple seconds accuracy. I strongly urge you to get a reliable NTP client for your PC. For years I’ve used Thinking Man Software’s Dimension 4, but a Google search will turn up several other NTP clients for Windows.
image: S Sepp, Wooden Hourglass 3, Wikimedia Commons