Date::Manip::TZ(3pm) User Contributed Perl Documentation Date::Manip::TZ(3pm)
NAME
Date::Manip::TZ - an interface to the time zone data
SYNOPSIS
use Date::Manip::TZ;
$tz = new Date::Manip::TZ;
Data for most (and hopefully all) time zones used around the world have
been gathered and is publicly available in the zoneinfo (or Olson)
database.
This module uses the data from the zoneinfo database to perform various
time zone operations.
DESCRIPTION
Every time zone has some of the following characteristics:
name
Every time zone has a unique name. In the zoneinfo database, these
are something similar to:
America/New_York
aliases
Time zones may have (but are not required to have) one or more
aliases. Each alias is unique, and is not the same as any time
zone name. An alias can be used in exactly the same way as a name.
periods
Every time zone is broken up into periods. Each period describes
how a portion of time relates to GMT, and how it might be
expressed.
Each period includes the following information:
start time, end time
The period begin and ends at certain times. The times are included
both as an absolute GMT time, and as a wall clock time. The wall
clock start time is the time that will be on a clock just as the
period starts (i.e. after a time change). The wall clock end time
is the time on a clock immediately before the period ends.
offset
The entire period has an offset which is how much the wall clock
time differs from GMT.
abbreviation
When expressing the time period, an abbreviation (such as EST) is
typically used.
daylight saving time flag
Every period is categorized as a standard time or a daylight saving
time. The flag will be 1 if it is a daylight saving time, or 0 if
it is a standard time.
Date::Manip includes all of the data for all of the time zones from the
zoneinfo database. This data is available from:
<ftp://ftp.iana.org/tz/>
Additional data from other standards are also used.
The zoneinfo database is not necessary in order to use Date::Manip.
Instead, all of that data has been extracted and stored in a series of
other modules which are used to handle each time zone. In that way,
Date::Manip has no dependency on any other source of data.
The Date::Manip::Zones document contains detailed information on the
data available.
METHODS
In all methods, the following variables are used:
$zone
This is a string which contains a valid time zone name. For
example:
America/New_York
$alias
This is a strings which contains a valid time zone name, or a valid
time zone alias. For example:
America/New_York
US/Eastern
EST5EDT
$abbrev
This is a string which contains a valid time zone abbreviation. For
example:
EST
$offset
This is a time zone entered as an offset. An offset is either a
string of one of the formats:
+HH
+HHMM
+HHMMSS
+HH:MM
+HH:MM:SS
or it can be a list reference:
[HH,MM,SS]
If a list reference is used, the sign must be included with all
values. So, the offset "-05:30" would be the list reference:
[-5,-30,0]
$dstflag
This is always one of the values: std, stdonly, dst, dstonly
It defaults to "std" if it is not present. When determining a time
zone, it is usually necessary to check a number of different time
zone and DST combinations.
If $dstflag is "std", it will check both standard and daylight
saving times, but will give preference to standard times. If
$dstflag is "stdonly", only standard times will be checked.
The "dst" flag will search both, but give preference to daylight
saving times. The "dstonly" values will only use daylight saving
times.
$date
A date is always a string containing a date in one of the formats:
YYYYMMDDHH:MN:SS
YYYY-MM-DD-HH:MN:SS
YYYYMMDDHHMNSS
or a list reference:
[Y,M,D,H,MN,S]
$isdst
This is 0 if a date is in standard time, 1 if it is in daylight
saving time.
$period
A period is a list reference currently containing the following
items:
[ $dateUT, $dateLT, $offsetstr, $offset, $abbrev, $isdst,
$endUT, $endLT, $begUTs, $begLTs, $endUTs, $endLTs ]
$dateUT and $dateLT are the starting date of the period (i.e. the
first second in a period) in universal (GMT) time and local (wall
clock) time. $endUT and $endLT are the end date of the period (i.e.
the last second in a period) in universal and local time. These
are all stored as list references.
$offsetstr is the string representation of the offset ("+05:00:00")
and $offset is the corresponding list reference form ([5,0,0]).
$abbrev is the abbreviation that applies during this period, and
$isdst is 0 or 1 if it is standard or daylight saving time.
When accessing the elements in a period, use ONLY positive indices.
In other words, to get $endUT, access it as $$period[6], NOT as
$$period[-2], since I am considering adding more information to the
period description that may speed up performance.
$begUTs is the string representation (YYYYMMDDHH:MN:SS) of $begUT.
Similar for $begLTs, $endUTs, and $endLTs.
The following methods are available:
base
config
err
new
new_config
Please refer to the Date::Manip::Obj documentation for these
methods.
all_periods
@periods = $tz->all_periods($zone,$year);
This returns the description of all time zone periods that occur
(in full or in part) during the given year. The year is measured
in universal (GMT) time.
convert
convert_to_gmt
convert_from_gmt
convert_to_local
convert_from_local
These functions convert a date from one time zone to another.
($err,$date,$offset,$isdst,$abbrev) =
$tz->convert($date,$from,$to [,$isdst]);
This converts a date in the time zone given by $from to the time
zone given by $to.
($err,$date,$offset,$isdst,$abbrev) =
$tz->convert_to_gmt($date [,$from] [,$isdst]);
This converts a date to GMT. If $from is given, it is the current
time zone of the date. If $from is omitted, it defaults to the
local time zone.
The value of $isdst returned is always 0.
($err,$date,$offset,$isdst,$abbrev) =
$tz->convert_from_gmt($date [,$to]);
This converts a date from GMT to another time zone. If $to is
given, the date is converted to that time zone. Otherwise, it is
converted to the local time zone.
($err,$date,$offset,$isdst,$abbrev) =
$tz->convert_to_local($date [,$from] [,$isdst]);
($err,$date,$offset,$isdst,$abbrev) =
$tz->convert_from_local($date [,$to] [,$isdst]);
Similar to the convert_to_gmt and convert_from_gmt functions. If
$from or $to are omitted, they default to GMT.
If there is any ambiguity about whether $date is in DST or not
(i.e. if it is a date that is repeated during a time change due to
the clock being moved back), the $isdst option can be passed in as
an argument (it should be 0 or 1) to say which time to use. It is
ignored in all cases where $date can be determined without that
information.
The $isdst value passed back is 1 if the converted date is in DST.
The $offset value passed back is a list reference containing the
offset from GMT. $abbrev passed back is the time zone abbreviation.
Error codes are:
0 No error
1 Invalid arguments
2 Invalid FROM zone
3 Invalid TO zone
4 Invalid date
curr_zone
$tz->curr_zone();
This returns the system time zone. The system time zone is
determined using the methods described below in the "DETERMINING
THE SYSTEM TIME ZONE" section.
This is the time zone that is used by default unless the SetDate or
ForceDate config variable is set to a different zone.
$tz->curr_zone(1);
This clears the system time zone and re-determines it using the
methods described below.
The main reason to do this is if the curr_zone_methods method is
used to change how the time zone is determined.
curr_zone_methods
$tz->curr_zone_methods(@methods);
This sets the list and order of methods to use in determining the
local time zone. The various methods available are listed below in
the section "DETERMINING THE SYSTEM TIME ZONE".
Some methods may require one or more arguments. For example, the
method named "mainvar" takes an option that is the name of a
variable. The arguments must be included in the @methods list
immediately after the method name (so @methods is actually a
mixture of method names and arguments).
This method may not be used in any environment where taint checking
is enabled. If it is, it will issue a warning, but will NOT change
the method list.
date_period
$period = $tz->date_period($date,$zone,$wall_clock [,$isdst]);
This returns the period information for the given date. $date
defaults to GMT, but may be given as local (i.e. wall clock) time
if $wall_clock is non-zero. The period information is described in
the periods method below.
If a wall clock time is given, no period is returned if the wall
clock time doesn't ever appear (such as when a time change results
in the clock moving forward "skipping" a period of time). If the
wall clock time appears twice (i.e. when a time change results in
the clock being set back), the $isdst variable is used. The
standard time is used unless $isdst is non-zero. $isdst is ignored
except in the case where there are two possible periods.
define_abbrev
($err,$val) = $tz->define_abbrev($abbrev,@zone);
When encountering an abbreviation, by default, all time zones which
ever include the abbreviation will be examine in the order given in
the Date::Manip::Zones manual.
Occasionally, it may be necessary to change the order. This is true
if you are parsing dates in a time zone which uses an abbreviation
which is also used in another time zone, and where the other time
zone is given preference. As an example, the abbreviation "ADT"
will default to the "Atlantic/Bermuda" time zone. If you are in the
"America/Halifax" time zone (which also uses that abbreviation),
you may want to change the order of time zones.
This will take an abbreviation (which must be a known
abbreviation... there is no means of defining a totally new
abbreviation) and a list of zones. This will set the list of zones
that will be checked, and the order in which they are checked, when
a date is encountered with the given abbreviation. It is not
necessary that the list include every zone that has ever used the
abbreviation, but it may not include a zone that has never used it.
If $abbrev is "reset", all abbreviations are reset to the standard
values. If @zone includes only the element 'reset', the default
list for $abbrev is restored.
The following error codes are returned:
0 No error
1 $abbrev is not a valid abbreviation in any time zone
2 A zone (returned as $val) is not a valid time zone
3 A zone (returned as $val) does not use the abbreviation
For more information about the different zones which may correspond
to each abbreviation, and the order in which they will be examined
by default, refer to the Date::Manip::Zones manual.
define_alias
$err = $tz->define_alias($alias,$zone);
This will define a new alias (or override an existing alias). $zone
must be a valid zone or an error is returned.
For more information about the different aliases which are set by
default, refer to the Date::Manip::Zones manual.
If $alias is "reset", all aliases will be reset to the standard
values. If $zone is "reset", $alias will be reset to the standard
value.
define_offset
($err,$val) = $tz->define_offset($offset, [$dstflag,] @zone);
This is similar to the define_abbrev method. When an offset is
encountered, all time zones which have ever included that offset
are checked. This will defined which time zones, and in what order,
they should be checked.
The zones to both standard and daylight saving times which include
the offset (if $dstflag is "std" or "dst") or to only one or the
other.
If $offset is "reset", all lists are reset to the default values.
If @zone includes only the element 'reset', the default list and
order is restored for $offset ($dstflag must not be given).
The following error codes are returned:
0 No error
1 $offset is not a valid offset in any time zone
2 $offset is not a valid offset in the selected
time (if doing "dstonly" or "stdonly")
3 A zone (returned as $val) is not a valid time zone
4 A zone (returned as $val) does not use the offset
5 A zone (returned as $val) does not include the
offset in the selected time (if doing "dstonly"
or "stdonly")
9 Offset is not a valid offset
periods
@periods = $tz->periods($zone,$year);
This returns the description of all time zone periods that begin
during the year given. The year is measured in universal (GMT)
time.
If no time zone period starts in the given year, nothing is
returned.
@periods = $tz->periods($zone,undef,$year);
This returns all periods that begin in any year from 0001 to $year.
@periods = $tz->periods($zone,$year0,$year1);
This returns all periods that begin in any year from $year0 to
$year1.
tzdata
tzcode
$vers = $tz->tzdata();
$vers = $tz->tzcode();
These return the versions of the tzdata and tzcode packages used to
generate the modules.
zone
$zone = $tz->zone(@args);
@zone = $tz->zone(@args);
This function will return a list of all zones, or the default zone,
which matches all of the supplied information. In scalar context,
it will return only the default zone. In list context, it will
return all zones.
@args may include any of the following items, and the order is not
important.
A zone name or alias ($alias)
A zone abbreviation ($abbrev)
An offset ($offset)
A dstflag ($dstflag)
A date ($date)
It is NOT valid to include two of any of the items. Any time zone
returned will match all of the data supplied.
If an error occurs, undef is returned. If no zone matches, an empty
string, or an empty list is returned.
The order of the zones will be determined in the following way:
If $abbrev is given, the order of time zones will be determined by
it (and $dstflag). If $dstflag is "std", all zones which match
$abbrev in standard time are included, followed by all that match
$abbrev in saving time (but no duplication is allowed). The reverse
is true if $dstflag is "dst".
If $abbrev is not given, but $offset is, $offset (and $dstflag)
will determine the order given. If $dstflag is "std", all zones
which match $offset in standard time are included, followed by all
that match $offset in saving time (but no duplication is allowed).
The reverse is true if $dstflag is "dst".
If $date is given, only zones in which $date will appear in a zone
that matches all other information are given. $date is a wall clock
time.
If no $zone, $abbrev, or $offset are entered, the local time zone
may be returned (unless $date is entered, and it doesn't exist in
the local time zone).
NOTE: there is one important thing to note with respect to $dstflag
when you are working with a timezone expressed as an offset and a
date is passed in. In this case, the default value of $dstflag is
"dst" (NOT "stdonly"), and you probably never want to pass in a
value of "std" (though passing in "stdonly" is okay).
For standard offsets (with no minute component), there is always a
standard timezone which matches that offset. For example, the
timezone "+0100" matches the timezone "Etc/GMT+01", so you will
never get a timezone in daylight saving time if $dstflag is "std".
If you want to pass in a date of 2001-07-01-00:00:00 and an
timezone of "+0100" and you want to get a timezone that refers to
that date as a daylight saving time date, you must use the $dstflag
of "dst" (or "dstonly").
Because this is almost always the behavior desired, when a zone is
passed in as an offset, and a date is passed in, the default
$dstflag is "dst" instead of "std". In all other situations, the
default is still "std".
If the timezone is expressed as an abbreviation, this problem does
not occur.
TIME ZONE INFORMATION IN DATE::MANIP
Date::Manip makes use of three potentially different time zones when
working with a date.
The first time zone that may be used is the actual local time zone.
This is the time zone that the computer is actually running in.
The second time zone is the working time zone. Usually, you will want
the default time zone to be the local time zone, but occasionally, you
may want the default time zone to be different.
The third time zone is the actual time zone that was parsed, or set,
for a date. If a date contains no time zone information, it will
default to the working time zone.
The local time zone is determined using the methods described in the
following section. Methods exist for locating the zone in one of the
system configuration files, determining it by running a system command,
or by looking it up in the registry (for Windows operating systems).
If all of these methods fail, the local time zone may be set using
either the $::TZ or $ENV{TZ} variables. Please note that these should
ONLY be used to set the actual local time zone.
If you are running in one time zone, but you want to force dates to be
specified in an alternate time zone by default, you need to set the
working time zone. The working time zone defaults to the local time
zone, but this can be changed using either the SetDate or ForceDate
config variables. Refer to the Date::Manip::Config manual for more
information.
Finally, when a date is actually parsed, if it contains any time zone
information, the date is stored in that time zone.
DETERMINING THE SYSTEM TIME ZONE
There are a large number of ways available for determining the time
zone. Some or all of them may be checked. A list of methods to use is
provided by default, and may be overridden by the curr_zone_methods
function described above. To override the default order and/or list of
methods, just pass in a list of method names (with arguments where
necessary), and only those methods will be done, and in the order
given.
The following methods are available:
Method Argument(s) Procedure
====== =========== =========
main VAR The main variable named VAR is
checked. E.g. "main TZ" checks
the variable $::TZ .
env TYPE VAR The named environment variable
is checked and the type of
data stored there (TYPE can
be 'zone' or 'offset' which
is the number of seconds from
UTC).
file FILE Look in the given file for any
one of the following case
insensitive lines:
ZONE
tz = ZONE
zone = ZONE
timezone = ZONE
ZONE may be quoted (single or
double) and whitespace is
ignored (except that underscores
in the zone name may be replaced
by whitespace on some OSes). If
the entire line is a zone, it must
be the first non-blank non-comment
line in the file.
command COMMAND Runs a command which produces
a time zone as the output.
cmdfield COMMAND N Runs a command which produces
whitespace separated fields,
the Nth one containing the
time zone (fields are numbered
starting at 0, or from the
end starting at -1).
gmtoff Uses the current offset from
GMT to come up with a best guess.
tzdata FILE DIR This uses a system config file that
contains a pointer to the local tzdata
files to determine the timezone. On
many operating systems, use:
tzdata /etc/localtime /usr/share/zoneinfo
FILE is the system file. DIR is the
directory where the tzdata files are stored.
The config file is either a link to a file
in the tzdata directory or a copy of one
of the files.
registry Look up the value in the
Windows registry. This is only
available to hosts running a
Windows operating system.
Note that the "main" and "env" methods should only be used to specify
the actual time zone the system is running in. Use the SetDate and
ForceDate config variables to specify an alternate time zone that you
want to work in.
By default, the following methods are checked (in the order given) on
Unix systems:
main TZ
env zone TZ
file /etc/TIMEZONE
file /etc/timezone
file /etc/sysconfig/clock
file /etc/default/init
command "/bin/date +%Z"
command "/usr/bin/date +%Z"
command "/usr/local/bin/date +%Z"
cmdfield /bin/date -2
cmdfield /usr/bin/date -2
cmdfield /usr/local/bin/date -2
command "/bin/date +%z"
command "/usr/bin/date +%z"
command "/usr/local/bin/date +%z"
tzdata /etc/localtime /usr/share/zoneinfo
gmtoff
The default methods for Windows systems are:
main TZ
env zone TZ
registry
gmtoff
The default methods for VMS systems are:
main TZ
env zone TZ
env zone SYS$TIMEZONE_NAME
env zone UCX$TZ
env zone TCPIP$TZ
env zone MULTINET_TIMEZONE
env offset SYS$TIMEZONE_DIFFERENTIAL
gmtoff
The default methods for all other systems are:
main TZ
env zone TZ
gmtoff
If anyone wants better support for a specific OS, please contact me and
we'll coordinate adding it.
In all cases, the value returned from the method may be any of the
following:
the full name of a time zone (e.g. America/New_York)
or an alias
an abbreviation (e.g. EDT) which will be used to
determine the zone if possible
an offset (+hh, +hhmn, +hh:mm, +hh:mm:ss) from GMT
The Date::Manip::Zones module contains information about the time zones
and aliases available, and what time zones contain the abbreviations.
DESIGN ISSUES
The design decisions made in writing this module may cause some
questions (and probably complaints). The time zone modules are all
generated using scripts (included in the Date::Manip distribution)
which use the standard tzdata tools to parse the tzdata files and store
that information in perl modules.
I'd like to address some of them, to avoid answering some of the "why
did you do it that way" remarks. I do welcome discussion about these
decisions... but preferably after you understand why those decisions
were made so that that we have an informed basis to begin a discussion.
Why not use existing zoneinfo files
Some people will probably think that I should have written an
interface to the zoneinfo files which are distributed with most
operating systems. Although I considered doing that, I rejected
the idea for two reasons.
First, not all operating systems come with the zoneinfo databases
in a user accessible state (Microsoft for example). Even those
that do include them store the information in various formats and
locations. In order to bypass all that, I have included the data
directly in these modules.
Second, as I was doing my initial investigations into this, I ran
into a bug in the Solaris zoneinfo tools (long since fixed I'm
sure). I decided then that I didn't want to depend on an
implementation where I could not control and fix the bugs.
Why not use the native tzdata files
Another decision people may question is that I parse the tzdata
files and store the data from them in a large number of perl
modules instead of creating an interface to the tzdata files
directly. This was done solely for the sake of speed. Date::Manip
is already a slow module. I didn't want to slow it down further by
doing the complex parsing required to interpret the tzdata files
while manipulating dates. By storing the data in these modules,
there is little or no parsing done while using Date::Manip modules.
It costs a little disk space to store this information... but very
little of it is actually loaded at runtime (time zone data is only
loaded when the time zone is actually referred to), so I feel it's
a good tradeoff.
Why store the information in so many files
The data from the native tzdata files are parsed and stored in two
sets of modules. These include almost 500 Date::Manip::Offset::*
modules and almost 450 Date::Manip::TZ::* modules.
I note that on my linux box, /usr/share/zoneinfo (which contains
data files generated from the tzdata files) contains over 1700
files, so I'm not doing anything "new" by breaking up the
information into separate files. And doing so has a huge impact on
performance... it is not necessary to load and/or manipulate data
from time zones which are not in use.
The minute I made the decision to distribute the timezone
information myself, as opposed to using the system version, it was
a given that there would be a lot of files.
These modules are loaded only when the time zone or offset is
actually used, so, unless dates from around the world are being
parsed, only a very small number of these modules will actually be
loaded. In many applications, only a single TZ module will be
loaded. If parsing dates which have timezone information stored as
offsets, one or two Offset modules will also be loaded.
The disk space seems excessive
Currently, the disk usage of the perl files is around 9 MB. Total
disk usage for /usr/share/zoneinfo on my computer is around 4 MB.
There are a couple of differences.
The primary difference is that the zoneinfo files are stored in a
binary (and hence, more compressed) version, where the perl modules
have all the data in pure text.
Since these are all automatically generated and used, it may be
beneficial to store the data in some packed binary format instead
of the fully expanded text form that is currently in use. This
would decrease the disk space usage, and might improve performance.
However, the performance improvement would happen only once per
timezone, and would make for more complicated code, so I'm not very
interested in pursuing this.
Another aspect of the current modules is that they all include pod
documentation. Although not necessary, this allows users to easily
see what modules handle which time zones, and that's nice. It also
allows me to use pod_coverage tests for the module which is a nice
check to make sure that the documentation is accurate.
All told, I don't consider the disk usage excessive at all.
KNOWN PROBLEMS OR ISSUES
Unable to determine Time Zone
When using Date::Manip, when the module is initialized, it must be
able to determine the local time zone. If it fails to do so, an
error will occur:
Unable to determine Time Zone
and the script will exit.
In the past, this was the most common problem with using
Date::Manip . With the release of 6.00, this problem should be
significantly less common. If you do get this error, please refer
to the section above DETERMINING THE SYSTEM TIME ZONE for
information about determining the local time zone. I am also
interested in hearing about this so that I can update the default
list of methods to be able to determine the local time zone better.
Asia/Jerusalem time zone
The Asia/Jerusalem time zone has a non-standard way of specifying
the start and end of Daylight Saving Time based on the Hebrew
calendar.
As a result, there is no way to specify a simple rule to define
time zone changes for all years in the future. As such, this module
supports all time zone changes currently specified in the zoneinfo
database (which currently goes to the year 2037) but does not
attempt to correctly handle zone changes beyond that date. As a
result, Date::Manip should not be used to parse dates in the
Jerusalem time zone that are far enough in the future that
information is not included in the current version of the zoneinfo
database.
LMT and zzz abbreviations
Both the LMT and zzz abbreviations are used in the zoneinfo
databases. LMT is use for most time zones for the times before the
Gregorian calendar was adopted, and zzz is used for a few where the
time zone was created and no description of dates prior to that are
supported. Both LMT and zzz are basically ignored in parsing dates
(because there is no reasonable way to determine which zone they
are referring to), and will be treated as the local time zone
regardless.
KNOWN BUGS
None known.
BUGS AND QUESTIONS
Please refer to the Date::Manip::Problems documentation for information
on submitting bug reports or questions to the author.
SEE ALSO
Date::Manip - main module documentation
LICENSE
This script is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
AUTHOR
Sullivan Beck (sbeck@cpan.org)
perl v5.36.0 2023-03-05 Date::Manip::TZ(3pm)
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