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

Jim Biard jbiard at cicsnc.org
Tue Jun 30 10:29:37 MDT 2015


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
    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,

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