[tz] What's "right"?

Tony Finch dot at dotat.at
Mon Nov 16 23:31:46 UTC 2020

Brooks Harris <brooks at edlmax.com> wrote:
> While the Posix time specification is somewhat vague (and, as I understand it,
> *intentionally* vague to accommodate legacy systems) the NTP specification is
> more clear.

To a limited extent. NTP is all about distributing the current time so the
exact meaning of historical timestamps is largely irrelevant. NTP has the
same issue as POSIX time that it skips leapseconds: there is a fixed
numerical offset between NTP and POSIX time, stated in RFC 5905 figure 4:

   | 1 Jan 1970  | 40,587     | 0   | 2,208,988,800 | First day UNIX   |

   | 31 Dec 1999 | 51,543     | 0   | 3,155,587,200 | Last day 20th    |
   |             |            |     |               | Century          |

$ TZ=UTC date -d @$((3155587200 - 2208988800)) +%FT%TZ

So, like POSIX time, you can't tell which UTC time corresponds to an NTP
timestamp around a leapsecond without some extra information. The
definition of POSIX time makes it clear that a positive leap second gets
the same time_t value as the following second, but NTP does not specify
how leapseconds are labelled to that level of detail.

The NTP wire protocol has two leap second warning bits, but their values
are not synchronized to the leap second, so they cannot be used to
disambiguate the timestamp. Insted they relate to how the NTP daemon and
the kernel clocks communicate state: the NTP leap second warning bits
correspond to the ntp_adjtime() STA_INS and STA_DEL flags, but to
disambiguoate timestamps you need the return values TIME_OOP or TIME_WAIT
which are not present on the wire. An NTP server will continue returning
the same leap warning flags through the leap second (while ntp_adjtime
would return TIME_OOP or TIME_WAIT) until the next time it adjusts the
clock and resets the leap state.


The upshot is that you can't use NTP as a good example of how to label

f.anthony.n.finch  <dot at dotat.at>  http://dotat.at/
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