Yes, TAI is an epochal time, number of seconds since a reference time. In EOSDIS, folks often use TAI93, number of seconds since 01 January 1993 00:00:00.
We have tools that translate between TAI93 and UTC, which rely on an external file (leapsec.dat) to compute the time conversions.
On Aug 23, 2011, at 8:47 AM, Jim Biard wrote:
> Hi.
>
> According to the almighty Wikipedia ;), UTC is "a time standard based on International Atomic Time (TAI) with leap seconds added at irregular intervals to synchronize with the Earth's rotation." So TAI doesn't attempt to stay synchronized with the Earth's rotation.
>
> Another quote from the Wikipedia article on UTC states
> UTC is a discontinuous timescale, so it is not possible to compute the exact time interval elapsed between two UTC timestamps without consulting a table that describes how many leap seconds occurred during that interval. Therefore, many scientific applications that require precise measurement of long (multi-year) intervals use TAI instead.
> I'm not advocating for anything, just contributing some factoids.
>
> Grace and peace,
>
> Jim Biard
>
> On 8/23/2011 8:13 AM, Lynnes, Christopher S. (GSFC-6102) wrote:
>> On Aug 22, 2011, at 6:36 PM, John Caron wrote:
>>
>>
>>> On 8/22/2011 6:37 AM, Jonathan Gregory wrote:
>>>
>>>> Dear Chris
>>>>
>>>>
>>>>> Perhaps there could be an attribute we could set that says whether we have accounted for leap seconds? With the absence of such an attribute to be presumed as meaning leap seconds have been ignored.
>>>>>
>>>> Perhaps the real-world calendars with and without leap seconds should be
>>>> regarded as two different calendars, since they have different encodings
>>>> (meaning decoding/encoding as YMD HMS<-> time-interval since reference-time).
>>>> The "true" real-world calendar is the one with leap seconds.
>>>>
>>>> CF has a calendar
>>>> proleptic_gregorian
>>>>
>>>> A Gregorian calendar extended to dates before 1582-10-15. That is, a year is a leap year if either (i) it is divisible by 4 but not by 100 or (ii) it is divisible by 400.
>>>>
>>>> What if we clarified this calendar as not having leap seconds? Then it could
>>>> be used for real-world applications for recent dates meaning that it was just
>>>> like the real world except that it doesn't have leap seconds.
>>>>
>>>> Model calendars, which are already idealised wrt length of year, don't have
>>>> leap seconds anyway, I am sure.
>>>>
>>>> Best wishes
>>>>
>>>> Jonathan
>>>> _______________________________________________
>>>> CF-metadata mailing list
>>>>
>>>> CF-metadata at cgd.ucar.edu
>>>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>>> I agree that a separate calendar is needed if we want to have leap
>>> seconds. I think the common form is UTC (or TAI?). Chris, what does the
>>> satellite community use?
>>>
>> Both UTC and TAI, actually.
>>
>>
>>> _______________________________________________
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>>>
>>> CF-metadata at cgd.ucar.edu
>>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>> Christopher Lynnes
>> Goddard Earth Sciences Data and Information Center, NASA/GSFC
>> 301-614-5185
>>
>> _______________________________________________
>> CF-metadata mailing list
>>
>> CF-metadata at cgd.ucar.edu
>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
> --
> Jim Biard
>
> Government Contractor, STG Inc.
> Remote Sensing and Applications Division (RSAD)
> National Climatic Data Center
> 151 Patton Ave.
> Asheville, NC 28801-5001
>
>
> jim.biard at noaa.gov
>
> 828-271-4900
>
> _______________________________________________
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--
Dr. Christopher Lynnes NASA/GSFC, Code 610.2 phone: 301-614-5185
Received on Tue Aug 23 2011 - 07:36:21 BST
