Leap Second

Page HhMmSs talks about these, but LeapSeconds are rare, and unless you need accuracy to seconds, of miniscule effect in the scheduling of meetings, travel reimbursement receipts, flight schedules, ...

Time is the fractional part of a day -- as in YyyyMmDd.HhMmSs

Astronomers often need to keep track of these things for the timing of eclipses and occultations. Astronomers use the JulianDate? calendar, which has its epoch at January 1, 4713 BCE, GMT. They then measure days and/or seconds since.

They seem to only happen on the last day of June (Yyyy0630) or December (Yyyy1231) and from the site below they are almost always going to make one minute of one of those days say that the clock goes from .235959 to .235960 before its normal passing to the next month .000000 - the rules allow for .235961 as well and for sometimes skipping .235959 altogether when the rules of physics change how the earth rotates.

Using YyyyMmDd.HhMmSs in GMT, the first LeapSecond was clocked as such: 19720630.235959 19720630.235960 19720701.000000

http://tf.nist.gov/general/leaps.htm http://tf.nist.gov/pubs/bulletin/leapsecond.htm

The next LeapSecond will look like this: 20081231.235959 20081231.235960 20090101.000000 HappyNewYear!

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Leap seconds are needed so that users of the astronomical time scale (UT1) can use UTC and know that the difference between the two time scales is never greater than 0.9 seconds. Currently, the difference between UT1 and UTC is changing at a rate that requires the addition of less than one leap second per year. When a leap second is necessary, an announcement is made at least several months in advance, and all leap seconds so far have been implemented on either June 30th or December 31st.

• Leap Second FAQ
  • http://tf.nist.gov/general/leaps.htm