On 2014-01-17 05:22 PM, Zefram wrote:
Brooks Harris wrote:
Yes, I understand that. Perhaps using the word "origin" was careless.
Maybe you can suggest a better term.
"proleptic". You may usefully add "with astronomical year numbering" to
make clear that zero and negative year numbers are valid. But really,
when you're defining a time scale, the calendar is irrelevant. It's a
separate concern that should be addressed separately.
Of course the idea is that dates after 1972-01-01T00:00:00Z are
"earth corrected" (Leap Seconds).
Are you implying that dates before are not? That wouldn't be a proleptic
UTC.
When I read this I suddenly realized what you seem to be objecting to -
that I'm calling it "proleptic UTC". We may need another name for this
portion of the CCT timescale.
Yes, I'm saying date-time on the scale previous to 1972-01-01T00:00:00Z
are not accurate, in the same way NTP and POSIX are not.
First, it sweeps away the non-integral Seconds complexity in the
historical record from 1958 (or maybe 1960-61, depending on how you
interpret what happened when) to 1972-01-01T00:00:00Z, in the same way
NTP and POSIX do.
Second, it extrapolates an uncompensated Gregorian calendar count into
the past to 0000-01-01T00:00:00Z (maybe its not legitimate to notate
that date-time as if it were a UTC date-time) and beyond into negative
uncompensated Gregorian calendar years. This is not actually relevant to
the purpose of time-keeping after 1972, but it makes the calendars
counting method consistent, avoiding the pesky 1970 "barrier" in the
POSIX spec.
Third, ten "proleptic Leap Seconds", or, "artificial Leap Seconds" are
in effect at 1900-01-01T00:00:00Z to support NTP's prime epoch. These
remain in effect until the start of the true UTC scale, at
1972-01-01T00:00:00Z, and this spans 1970-01-01T00:00:00Z, the POSIX
origin (the Epoch), where 10 Leap Seconds must also be in effect.
So, no, its not really "proleptic UTC" - its a Seconds scale extending
back from 1972 with an uncompensated Gregorian calendar count and
artificial, proleptic "Leap Seconds" from 1900-01-01T00:00:00Z. Its
constructed to facilitate computational convenience with respect to NTP,
POSIX, and 1588/PTP.
-Brooks
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