Hi Gerard, On 2015-03-07 12:04 PM, G Ashton wrote:
Brooks Harris wrote on Saturday, March 7, 2015 11:50 : . . "The challenge I'm trying to solve is to provide a deterministic timekeeping and labeling scheme for date and time *after* 1972-01-01T00:00:00Z (UTC) = 1972-01-01T00:00:10 (TAI). This is essentially the purpose of "civil time" timekeeping as is typically intended....The timescale before 1972 is an abstract proleptic Gregorian calendar scale for purposes of calculation convenience. On this scale, like NTP, PTP, and POSIX, any date-time before 1972-01-01T00:00:00Z (UTC) is considered either inaccurate or invalid." Civil timekeeping is concerned with many things, including determining when one date ends and another begins. Thus civil timekeeping is inextricably linked to civil calendars. Although the time of day of past events become less and less important as the decades pass, the date of those events remain important. Since some computer applications routinely attempt, in their clumsy way, to account for timezones, timekeeping is potentially important for the computer representation of timestamps, even when the humans using the computer are only interested in the date. Of course, dates long before 1972 are of interest in civil matters; dates of birth immediately come to mind.
I agree.
So when Brooks Harris presents his API to his stakeholders, I think a more thorough explanation of why date-time expressions before 1972 will be " considered either inaccurate or invalid" will be needed.
It is typically warned that date and time before 1972 cannot be accurately represented with NTP or POSIX, for examples. These timescale's origins precede 1972-01-01T00:00:00Z (UTC) = 1972-01-01T00:00:10 (TAI) and seek to represent date-time counting *forward*. They give no consideration to date-time accuracy before 1972, but operate on proleptic scales convenient for computation. This is generally true with widely available timekeeping services on OSs, systems, languages, and many typical applications because so many of them implement mechanisms based on the heritage of the POSIX timekeeping mechanisms, complete with its flaws with respect to representing UTC and Leap Seconds.
In the discussions I've been involved with many people argued strenuously "we don't care about the past, only accurate date-time going forward!". The reason I'm choosing to ignore the subject of accurate date-times before 1972 is not that its not important, but probably the same reason its side-stepped by NTP, PTP, POSIX, and GPS - its just too expansive a topic to tackle in some commonly accepted way. For date-time before 1972 you've got to switch to some other timescale depending on the purpose at hand.
-Brooks
Gerard Ashton _______________________________________________ LEAPSECS mailing list LEAPSECS@leapsecond.com https://pairlist6.pair.net/mailman/listinfo/leapsecs
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