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Re: Representing UTC time in a readable format
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Re: Representing UTC time in a readable format


  • Subject: Re: Representing UTC time in a readable format
  • From: Creed Erickson <email@hidden>
  • Date: Sat, 5 Jun 2004 11:28:24 -0700

On Saturday, June 5, 2004, at 03:08 AM, Malte Tancred wrote:

Are you saying that "UTC" defines how a date and time is to be presented visually, e.g. as in the string "2004-05-23T13:15:27Z"?

That was my intended meaning.

I might of course be totally misinformed, but to the best of my knowledge UTC is as much a time scale as TAI is, the only difference being the periodic insertion of leap seconds in UTC.

I did a little more research and found *I* was misinformed. I had been under the assumption that UTC and ISO-8601 were synonymous. They are not, UTC *IS* defined as a time scale. In that sense I was wrong and stand corrected. Terribly sorry for any confusion I created.

According to the U.S. Naval Observatory and the National Institute of Standards and Technology, UTC has a leap second added any time it becomes more than 0.9 seconds behind UT1, by internationally agreed definition. UT1 is true astronomical time. I.E., UTC is explicitly leap second corrected. (I reference U.S.A. standards institutes because they are the most familiar: http://aa.usno.navy.mil/faq/docs/UT.html and http://www.boulder.nist.gov/timefreq/general/pdf/1383.pdf )

An internet enabled Mac OS X system is likely to be synchronized to an NTP server, and NTP servers usually try to use standards traceable to UTC. Therefore, an NSDate should be capable of returning proper UTC. This points out a bug in Apple's documentation. Specifically, the note re: UTC and TAI in the date and time concepts doc at http://developer.apple.com/documentation/Cocoa/Conceptual/ DatesAndTimes/Concepts/Dates.html is wrong.


-- Creed
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References: 
 >Re: Representing UTC time in a readable format (From: Malte Tancred <email@hidden>)

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