xref: /freebsd/contrib/tzcode/zdump.8 (revision 75411d157232ee3b4789b92c9205453e7d59a3d2)

.\" This file is in the public domain, so clarified as of
.\" 2009-05-17 by Arthur David Olson.
.Dd December 15, 2022
.Dt ZDUMP 8
.Os
.Sh NAME
.Nm zdump
.Nd timezone dumper
.Sh SYNOPSIS
.Nm
.Op Fl -help
.Op Fl -version
.Op Fl ivV
.Oo
.Fl c
.Op Ar loyear , Ns
.Ar hiyear
.Oc
.Oo
.Fl t
.Op Ar lotime , Ns
.Ar hitime
.Oc
.Op Ar timezone ...
.Sh DESCRIPTION
The
.Nm
program prints the current time in each
.Ar timezone
named on the command line.
.Pp
The following options are available:
.Bl -tag -width indent
.It Fl -version
Output version information and exit.
.It Fl -help
Output short usage message and exit.
.It Fl i
Output a description of time intervals.
For each
.Ar timezone
on the command line, output an interval-format description of the
timezone.
See
.Sx "INTERVAL FORMAT"
below.
.It Fl v
Output a verbose description of time intervals.
For each
.Ar timezone
on the command line,
print the times at the two extreme time values,
the times (if present) at and just beyond the boundaries of years that
.Xr localtime 3
and
.Xr gmtime 3
can represent, and
the times both one second before and exactly at
each detected time discontinuity.
Each line is followed by
.Cm isdst= Ns Ar D
where
.Ar 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
.Cm gmtoff= Ns Ar N
if the given local time is known to be
.Ar N
seconds east of Greenwich.
.It Fl V
Like
.Fl v ,
except omit output concerning extreme time and year values.
This generates output that is easier to compare to that of
implementations with different time representations.
.It Fl c Oo Ar loyear , Oc Ns Ar hiyear
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.
Cutoffs are at the start of each year, where the lower-bound
timestamp is inclusive and the upper is exclusive; for example,
.Ql "-c 1970,2070"
selects transitions on or after 1970-01-01 00:00:00 UTC
and before 2070-01-01 00:00:00 UTC.
The default cutoff is
.Ql -500,2500 .
.It Fl t Oo Ar lotime , Oc Ns Ar hitime
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
.Ar timezone
determines whether the count includes leap seconds.
As with
.Fl c ,
the cutoff's lower bound is inclusive and its upper bound is exclusive.
.El
.Sh "INTERVAL FORMAT"
The interval format is a compact text representation that is intended
to be both human- and machine-readable.
It consists of an empty line,
then a line
.Dq "TZ=\fIstring\fP"
where
.Ar string
is a double-quoted string giving the timezone, a second line
.Dq "\*- \*- \fIinterval\fP"
describing the time interval before the first transition if any, and
zero or more following lines
.Dq "\fIdate time interval\fP",
one line for each transition time and following interval.
Fields are
separated by single tabs.
.Pp
Dates are in
.Ql "yyyy - mm - dd"
format and times are in 24-hour
.Ql "hh : mm : ss"
format where
.Ql "hh <24" .
Times are in local time immediately after the transition.
A
time interval description consists of a UT offset in signed
.Ql "\(+- 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.
.Pp
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
.Dq "-"
or is
.Dq "zzz".
.Pp
In double-quoted strings, escape sequences represent unusual
characters.
The escape sequences are \es for space, and \e", \e\e,
\ef, \en, \er, \et, and \ev with their usual meaning in the C
programming language.
E.g., the double-quoted string
\*(lq"CET\es\e"\e\e"\*(rq represents the character sequence \*(lqCET
"\e\*(rq.\""
.Pp
Here is an example of the output, with the leading empty line omitted.
(This example is shown with tab stops set far enough apart so that the
tabbed columns line up.)
.Bd -literal -offset indent
TZ="Pacific/Honolulu"
-	-	-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	HWT	1
1945-08-14	13:30	-0930	HPT	1
1945-09-30	01	-1030	HST
1947-06-08	02:30	-10	HST
.Ed
.Pp
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.
.Pp
Here are excerpts from another example:
.Bd -literal -offset indent
TZ="Europe/Astrakhan"
-	-	+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
.Ed
.Pp
This time zone is east of UT, so its UT offsets are positive.
Also,
many of its time zone abbreviations are omitted since they duplicate
the text of the UT offset.
.Sh LIMITATIONS
Time discontinuities are found by sampling the results returned by
.Xr localtime 3
at twelve-hour intervals.
This works in all real-world cases;
one can construct artificial time zones for which this fails.
.Pp
In the
.Fl v
and
.Fl V
output,
.Dq "UT"
denotes the value returned by
.Xr gmtime 3 ,
which uses UTC for modern timestamps and some other UT flavor for
timestamps that predate the introduction of UTC.
No attempt is currently made to have the output use
.Dq "UTC"
for newer and
.Dq "UT"
for older timestamps, partly because the exact date of the
introduction of UTC is problematic.
.Sh SEE ALSO
.Xr tzfile 5 ,
.Xr zic 8