bug#34075: 26.1; calc-time-zone result 2h too low for TZ east of GMT during DST

[David O'Shea]

calc-time-zone returns time zone offset 2 hours less than expected for

a time zone ahead of UTC (east of Greenwich) when daylight savings is

active and the time zone is not specified within Emacs and instead is

fetched from the system.

Procedure to reproduce on a CentOS Linux system where /etc/localtime is

a symlink to /usr/share/zoneinfo/Australia/Sydney and daylight savings

time is active (e.g. on 14/Jan/2019):

1. Start calculator:         M-x calc RET

2. Get the time zone offset: t Z

3. Convert it to hours:      3600 /

For this location and time of year (daylight savings active), the offset

should be 11 hours, but the result of the above is 9.

I get the same behaviour with Emacs 24.2 (also built by myself) and

Emacs 24.3.1 as shipped by CentOS (RPM version 24.3-20.el7_4).

The offset is correct when daylight savings time is not active.

The info node "(calc) Time Zones" shows these default time zone

definitions are available in the calculator:

>      YST  PST  MST  CST  EST  AST    NST    GMT   WET     MET    MEZ

>       9    8    7    6    5    4     3.5     0     -1      -2     -2

These are negated compared to the usual convention of positive offsets

being used for time zones ahead of UTC (east of Greenwich) as is

followed by the function current-time-zone.  It would seem to me then

that for consistency with calculator's convention I should actually be

getting a negative result from calc-time-zone, but I don't, and I

think that's related to the absolute value of the offset being wrong.

It seems that when I invoke calc-time-zone, math-get-calendar-tzinfo

is called to get the timezone offset, and it does this:

      (setq math-calendar-tzinfo

            (list (* 60 (abs (nth 0 tzinfo)))

                  (* 60 (nth 1 tzinfo)))))))

I think the call to abs probably successfully converts the operating

system's view of the timezone offset from negative to positive for

timezones west of Greenwich, but fails to account for timezones east

of Greenwich.

When I change that code to just invert the sign on the timezone:

      (setq math-calendar-tzinfo

            (list (* -60 (nth 0 tzinfo))

   (* 60 (nth 1 tzinfo)))))))

I get a result of -11 from the sequence of steps shown at the top of

this bug report.  I didn't check closely to make sure that inverting

the sign is actually the correct behaviour but it's at least somewhat

consistent with the other code in making the offset negative for a

timezone east of Greenwich, and seems to work for this specific case.

I'm not considering this bug to be about the fact that the timezone

offset sign is inverted compared to usual conventions - and not filing

a separate bug about that - because (a) that might just be personal

preference and (b) changing it would presumably break any local

changes users may have made to math-tzone-names, so I imagine it'd be

pretty tricky.

In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu)

 of 2019-01-14 built on <my machine, name redacted>

System Description: CentOS release 6.3 (Final)

Recent messages:

For information about GNU Emacs and the GNU system, type C-h C-a.

Welcome to the GNU Emacs Calculator!  Press ‘?’ or ‘h’ for help, ‘q’ to quit

t-

Configured using:

 'configure --prefix=/opt/emacs-26.1'

Configured features:

SOUND NOTIFY LIBSELINUX GNUTLS LIBXML2 ZLIB THREADS

Important settings:

  value of $LANG: en_US.utf8

  value of $XMODIFIERS: @im=none

  locale-coding-system: utf-8-unix

Major mode: Calculator

Minor modes in effect:

  tooltip-mode: t

  global-eldoc-mode: t

  electric-indent-mode: t

  menu-bar-mode: t

  file-name-shadow-mode: t

  global-font-lock-mode: t

  font-lock-mode: t

  auto-composition-mode: t

  auto-encryption-mode: t

  auto-compression-mode: t

  buffer-read-only: t

  line-number-mode: t

  transient-mark-mode: t

Load-path shadows:

None found.

Features:

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Memory information:

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