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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 AT cpan.org)
perl v5.32.1 2021-11-20 Date::Manip::TZ(3pm)
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