As another weekend draws to a close, I was wishing for a bit more time to Get Stuff Done.
And then I remembered, during a weekend earlier this summer, that’s exactly what happened.
Saturday June 30, 2012 contained a leap second, so that weekend was one second longer than all the rest!
Yes, a minute CAN contain 61 seconds, but it does not happen very often.
After the leap second occurred, I took a photograph of the Dell server console which runs this www.glassblower.info website, here is what I saw:
The Linux kernel wanted to make sure that I was aware of this extra-long minute at the end of June 30 2012 when the leap second was inserted. In addition to the console output, of course, this message also appeared in dmesg (the kernel message buffer command).
For anyone interested in learning more about leap seconds, I’d recommend beginning with this Wikipedia page which starts with this information:
|A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to keep its time of day close to the mean solar time. The most recent leap second was inserted on June 30, 2012 at 23:59:60 UTC.
The UTC time standard, which is widely used for international timekeeping and as the reference for civil time in most countries, uses the international system (SI) definition of the second, based on atomic clocks. Like most time standards, UTC defines a grouping of seconds into minutes, hours, days, months, and years. However, the duration of one mean solar day is slightly longer than 24 hours (86400 SI seconds). Therefore, if the UTC day were defined as precisely 86400 SI seconds, the UTC time-of-day would slowly drift apart from that of solar-based standards, such as Greenwich Mean Time (GMT) and its successor UT1. The purpose of a leap second is to compensate for this drift, by occasionally scheduling some UTC days with 86401 or 86399 SI seconds.
Specifically, a positive leap second is inserted between second 23:59:59 of a chosen UTC calendar date (the last day of a month, usually June 30 or December 31) and second 00:00:00 of the following date. This extra second is displayed on UTC clocks as 23:59:60. On clocks that display local time tied to UTC, the leap second may be inserted at the end of some other hour (or half-hour), depending on the local time zone.
A negative leap second would suppress second 23:59:59 of the last day of a chosen month, so that second 23:59:58 of that date would be followed immediately by second 00:00:00 of the following date. However, since the UTC standard was established, negative leap seconds have never been needed.
Because the Earth’s rotation speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and unpredictable. Insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service (IERS), when needed to ensure that the difference between the UTC and UT1 readings will never exceed 0.9 second. Between their adoption in 1972 and June 2012, 25 leap seconds have been scheduled, all positive.
Leap seconds can cause a variety of problems with computer systems. In fact, there was a Java Leap Second Bug which took down a number of websites during the transition from June 30 2012 to July 1 2012. This article in Wired Magazine entitled “Leap Second Bug Wreaks Havoc Across Web” gives you a pretty good idea what mayhem leap seconds can cause.
Added: June 30, 2015: during the last minute of June 2015, right before 00:00:00 UTC July 1, this appears on the Linux console, hope everybody used their extra second wisely: