[tz] zdump new option -i for easier-to-review output
eggert at cs.ucla.edu
Sun May 29 18:49:46 UTC 2016
Jon Skeet wrote:
> I'd be perfectly happy with zdump gaining more display options, but I think
> there's still huge benefit in deciding on one *canonical* format for
I looked into the format you suggested, along with the other comments noted and
formats I've seen elsewhere (e.g., Shanks), and came up with the attached
proposal for a "canonical" -i format for zdump, with the design goals being a
format that is unambiguous, easy to review, and compact. Although this format's
columns don't always line up, in general aligning columns appears to be
impractical (in the extreme case, year numbers might exceed 9999!), and I found
that unaligned columns make it easier to see glitches anyway. The proposed -i
format does not contain versioning information as that would complicate
For what it's worth, the -i format is about 10% the size of -v format, and is
about 53% the size of the format you proposed.
This proposal is incomplete, for several reasons. First, it doesn't address leap
seconds. Second, it doesn't abbreviate predicted futures into POSIX TZ strings;
fixing this would make the output significantly shorter. Third, there is no
infrastructure for verifying a distribution by checksumming its zdump -i output.
So the proposal is documented as being experimental in the attached patch, and I
haven't installed it on github yet. Of course zdump -v has all these problems as
well, so the proposal format wouldn't make these problems worse.
The first attachment consists of the revised man-page output; the second
attachment is the change to tzcode.
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ZDUMP(8) System Manager's Manual ZDUMP(8)
zdump - time zone dumper
zdump [ option ... ] [ zonename ... ]
Zdump prints the current time in each zonename named on the command
These options are available:
Output version information and exit.
-i (This option is experimental: its behavior may change in future
versions.) Output a description of time intervals. For each
zonename on the command line, output an interval-format
description of the zone. See "INTERVAL FORMAT" below.
-v Output a verbose description of time intervals. For each
zonename on the command line, print the time at the lowest
possible time value, the time one day after the lowest possible
time value, the times both one second before and exactly at each
detected time discontinuity, the time at one day less than the
highest possible time value, and the time at the highest
possible time value. Each line is followed by isdst=D where D
is positive, zero, or negative depending on whether the given
time is daylight saving time, standard time, or an unknown time
type, respectively. Each line is also followed by gmtoff=N if
the given local time is known to be N seconds east of Greenwich.
-V Like -v, except omit the times relative to the extreme time
values. This generates output that is easier to compare to that
of implementations with different time representations.
Cut off interval output at the given year(s). Cutoff times are
computed using the proleptic Gregorian calendar with year 0 and
with Universal Time (UT) ignoring leap seconds. The lower bound
is exclusive and the upper is inclusive; for example, a loyear
of 1970 excludes a transition occurring at 1970-01-01 00:00:00
UTC but a hiyear of 1970 includes the transition. The default
cutoff is -500,2500.
Cut off interval output at the given time(s), given in decimal
seconds since 1970-01-01 00:00:00 Coordinated Universal Time
(UTC). The zonename determines whether the count includes leap
seconds. As with -c, the cutoff's lower bound is exclusive and
its upper bound is inclusive.
This format is experimental: it may change in future versions.
The interval format is a compact text representation that is intended
to be both human- and machine-readable. It consists of a first line
"TZ=string" where string is a double-quoted string giving the zone
name, a second line "- - interval" describing the time interval before
the first transition if any, and zero or more following lines "date
time interval", one line for each transition time and following
interval. Fields are separated by single spaces.
Dates are in yyyy-mm-dd format and times are in 24-hour hh:mm:ss format
where hh<24. Times are in local time immediately after the transition.
A time interval description consists of a UT offset in signed +-hhmmss
format, a time zone abbreviation, and an isdst flag. An abbreviation
that equals the UT offset is omitted; other abbreviations are double-
quoted strings unless they consist of one or more alphabetic
characters. An isdst flag is omitted for standard time, and otherwise
is a decimal integer that is unsigned and positive (typically 1) for
daylight saving time and negative for unknown.
In times and in UT offsets with absolute value less than 100 hours, the
seconds are omitted if they are zero, and the minutes are also omitted
if they are also zero. Positive UT offsets are east of Greenwich. The
UT offset -00 denotes a UT placeholder in areas where the actual offset
is unspecified; by convention, this occurs when the UT offset is zero
and the time zone abbreviation begins with "-" or is "zzz".
In double-quoted strings, escape sequences represent unusual
characters. The escape sequences are \s for space, and \", \\, \f, \n,
\r, \t, and \v with their usual meaning in the C programming language.
E.g., the double-quoted string ""CET\s\"\\"" represents the character
sequence "CET "\".
Here is an example:
- - -103126 LMT
1896-01-13 12:01:26 -1030 HST
1933-04-30 03 -0930 HDT 1
1933-05-21 11 -1030 HST
1942-02-09 03 -0930 HDT 1
1945-09-30 01 -1030 HST
1947-06-08 02:30 -10 HST
Here, local time begins 10 hours, 31 minutes and 26 seconds west of UT,
and is a standard time abbreviated LMT. Immediately after the first
transition, the date is 1896-01-13 and the time is 12:01:26, and the
following time interval is 10.5 hours west of UT, a standard time
abbreviated HST. Immediately after the second transition, the date is
1933-04-30 and the time is 03:00:00 and the following time interval is
9.5 hours west of UT, is abbreviated HDT, and is daylight saving time.
Immediately after the last transition the date is 1947-06-08 and the
time is 02:30:00, and the following time interval is 10 hours west of
UT, a standard time abbreviated HST.
Here are excerpts from another example:
- - +031212 LMT
1924-04-30 23:47:48 +03
1930-06-21 01 +04
1981-04-01 01 +05 1
1981-09-30 23 +04
2014-10-26 01 +03
2016-03-27 03 +04
This time zone is east of UT, so its UT offsets are positive. Also,
many of its time zone abbreviations omitted since they duplicate the
text of the UT offset.
If multiple zones are present, their representations are separated by
Time discontinuities are found by sampling the results returned by
localtime at twelve-hour intervals. This works in all real-world
cases; one can construct artificial time zones for which this fails.
In the -v and -V output, "UT" denotes the value returned by gmtime(3),
which uses UTC for modern time stamps and some other UT flavor for time
stamps that predate the introduction of UTC. No attempt is currently
made to have the output use "UTC" for newer and "UT" for older time
stamps, partly because the exact date of the introduction of UTC is
newctime(3), tzfile(5), zic(8)
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