[CF-metadata] UTC, GPS, and the impending leap second

From: Jim Biard <jbiard>
Date: Tue, 30 Jun 2015 12:29:37 -0400

Hi.

Since we've been discussing UTC vs GPS time and we are about to
experience a leap second event about 8 hours from when I send this, I
thought it might be helpful to show what's about to go on.

UTC has used leap seconds since 1972 to synchronize timestamps to the
rotational motion of the Earth. A leap second is added to try to keep
the UTC clock readout within 1 second of midnight when the 0 degree
longitude crosses the physical midnight point in the Earth-Sun system.
They don't add or subtract anything from the passage of time in terms of
elapsed seconds. The leap second (similarly to a leap day) is an
instruction to make the seconds part of the HH:MM:SS clock readout
(timestamp) count up one higher than usual, so proper GPS and UTC clock
readouts this evening (from my time zone) will run like this:

    GPS Elapsed Time since 1980-01-06
            GPS timestamp
            UTC timestamp
            Comment
    weeks 1851
    seconds 262813 2015-07-01 00:00:13
            2015-06-30 23:59:58
            
    weeks 1851
    seconds 262814 2015-07-01 00:00:14
            2015-06-30 23:59:59
    weeks 1851
    seconds 262815
            2015-07-01 00:00:15
            2015-06-30 23:59:60 leap second
    weeks 1851
    seconds 262816 2015-07-01 00:00:16
            2015-07-01 00:00:00
    weeks 1851
    seconds 262817 2015-07-01 00:00:17
            2015-07-01 00:00:01


GPS, on the other hand, is actually a count of elapsed weeks and seconds
within a week since it's epoch of 1980-01-06 00:00:00 UTC. It can also
be represented as a timestamp, and should be converted to a timestamp
without considering leap seconds. This puts it 15 seconds ahead of UTC
now, and 16 seconds ahead in about 8 hours when the new UTC leap second
is applied.

Grace and peace,

Jim

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Received on Tue Jun 30 2015 - 10:29:37 BST