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.\" $FreeBSD: src/usr.sbin/zic/zdump.8,v 1.7.2.2 2003/03/11 22:31:35 trhodes Exp $
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.Dd December 8, 2013
.Dt ZDUMP 8
.Os
.Sh NAME
.Nm zdump
.Nd timezone dumper
.Sh SYNOPSIS
.Nm
.Op Fl vV
.Op Fl c Bo Ar loyear Ns , Bc Ns Ar hiyear
.Op Fl t Bo Ar lotime Ns , Bc Ns Ar hitime
.Op Ar zonename ...
.Sh DESCRIPTION
The
.Nm
utility prints the current time in each
.Ar zonename
named on the command line.
.Pp
The following options are available:
.Bl -tag -width indent
.It Fl v
For each
.Ar 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 ends with
.Em isdst=1
if the given time is Daylight Saving Time or
.Em isdst=0
otherwise.
.It Fl V
Like
.Fl 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.
.It Fl c Bo Ar loyear Ns , Bc Ns Ar hiyear
Cut off verbose output near the start of the given year(s).
By default,
the program cuts off verbose output near the starts of the years -500 and 2500.
.It Fl t Bo Ar lotime Ns , Bc Ns Ar hitime
Cut off verbose output at the start of the given time(s), given in decimal
seconds since 1970-01-01 00:00:00 UTC.
.El
.Sh LIMITATIONS
The
.Fl v
option may not be used on systems with floating-point
.Vt time_t
values that are neither float nor double.
.Pp
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 output,
.Dq UT
denotes the value returned by
.Xr 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
.Dq UTC
for newer and
.Dq UT
for older time stamps, partly because the exact date of the introduction of
UTC is problematic.
.Sh SEE ALSO
.Xr ctime 3 ,
.Xr tzfile 5 ,
.Xr zic 8