Brooks Harris wrote: > The whole purpose of TAI is >a "realization" of TT, right? TAI shields us (I mean us normal >computer people, not astronomers or cosmologists) from the details of >how TAI is maintained
TAI does not shield you from the lack of atomic clocks prior to 1955. [NTP and POSIX time] >I don't think so. Both are indeed counts of Seconds, and both have a >relationship to UTC time. For POSIX, see <http://pubs.opengroup.org/onlinepubs/009604499/basedefs/xbd_chap04.html>, section 4.14, defining "Seconds Since the Epoch" as A value that approximates the number of seconds that have elapsed since the Epoch. A Coordinated Universal Time name (specified in terms of seconds (tm_sec), minutes (tm_min), hours (tm_hour), days since January 1 of the year (tm_yday), and calendar year minus 1900 (tm_year)) is related to a time represented as seconds since the Epoch, according to the expression below. [...] tm_sec + tm_min*60 + tm_hour*3600 + tm_yday*86400 + (tm_year-70)*31536000 + ((tm_year-69)/4)*86400 - ((tm_year-1)/100)*86400 + ((tm_year+299)/400)*86400 [...] The divisions in the formula are integer divisions; that is, the remainder is discarded leaving only the integer quotient. It's defined as a transformation of a broken-down UTC timestamp, not (despite its name) as a count of seconds since some instant. NTP's definition, by contrast, does speak of counting seconds, but it doesn't count leap seconds. It counts 86400 per UTC day regardless of leaps, and so is also effectively just a transformation of broken-down UTC timestamps. Both NTP and POSIX time values are sometimes described as a "count of non-leap seconds", which similarly gets across the essential point that the leap second history doesn't influence the scalar<->UTC relationship. >It *is* associated with a "specific instant in time" - its is >2,272,060,800 Seconds before 1972-01-01T00:00:00Z. It's 2272060800 *non-leap* seconds, which is to say 26297 days regardless of leaps on those days, on a notional proleptic UTC. Because the proleptic UTC is purely notional, this specification doesn't yield a specific instant in time. Because the leap history doesn't affect the scalar<->UTC relationship, there is no need to define this notional proleptic UTC any further. >I must be misunderstanding what you're getting at here. I can't >believe our understanding of this is so completely different. You appear to have not examined how NTP and POSIX time values actually behave around leap seconds. -zefram _______________________________________________ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
