Re: clocks


Mills@UDEL.EDU
Sun, 3 Jan 88 21:33:13 EST


Mark,

Before 1 January 1972, when the international time scales were rationalized
and leap-seconds introduced, the time standards were corrected at appropriate
intervals to maintain time with respect to Earth rotation to within 100 ms
or so. This fact, along with the fact that atomic clocks were rare near the
beginning of the century, makes it meaningless to consider leaps before that
date. There have been maybe a dozen leap seconds since then, all positive;
however, I don't have the dates handy at the moment.

In 1980, when IEN-142 was published, and again in 1983, when its successor
was published, it didn't seem like such a big deal to tell time down to the
milliseconds, especially since the precision provided by the protocol was
only to the second. Since these documents were published, protocols and
measurement techniques have evolved to the point that events can be synchronized
with high reliability to within a couple of milliseconds over campus LANs and
within a couple of hundred milliseconds over typical Internet paths. Thus,
it comes as no surprise that the leap-second issue has now become important.
See RFC-956, -957 and -958 for further discussion on timetelling in the
Internet and a description of the Network Time Protocol (NTP) which makes
all this possible.

I considered the issue of whether to build in leap-second corrections for
NTP, at least back to their inception in 1972 and decided against it. For
just about any purpose, it is not necessary to order events with respect
to what I will call archival time to better than a second; however, when
that becomes necessary, the user would have to insert the leaps as necessary
and using archival information telling when they were made.

In fact, the time scale NTP uses really concides with International Atomic
Time (TA-NBS). After leap-second corrections TA-NBS becomes UTC-NBS as
broadcast. TA-NTP concided with TA-NBS just after the original correction
was inserted prior to 1 January 1972. The rest is history.

Dave



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