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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Time Zone Database Parser redirect</title>
<META http-equiv="refresh" content="5;URL=http://howardhinnant.github.io/date/tz.html">
<style>
p {text-align:justify}
li {text-align:justify}
blockquote.note
{
background-color:#E0E0E0;
padding-left: 15px;
padding-right: 15px;
padding-top: 1px;
padding-bottom: 1px;
}
ins {color:#00A000}
del {color:#A00000}
code {white-space:pre;}
</style>
</head>
<body>
<center>
The contents you are looking for have moved. You will be redirected to the new
location automatically in 5 seconds. Please bookmark the correct page at
<a href="http://howardhinnant.github.io/date/tz.html">http://howardhinnant.github.io/date/tz.html</a>
</center>
<!--
<address align=right>
<br/>
<br/>
<a href="mailto:[email protected]">Howard E. Hinnant</a><br/>
2016-04-09<br/>
<a rel="license" href="http://creativecommons.org/licenses/by/4.0/"> <img alt="Creative
Commons License" style="border-width:0"
src="http://i.creativecommons.org/l/by/4.0/80x15.png" /></a><br /> This work is licensed
under a <a rel="license" href="http://creativecommons.org/licenses/by/4.0/">Creative
Commons Attribution 4.0 International License</a>.
</address>
<hr/>
<h1 align=center>Time Zone Database Parser</h1>
<h2>Contents</h2>
<ul>
<li><a href="#Introduction">Introduction</a></li>
<li><a href="#Overview">Overview</a></li>
<li><a href="#Synopsis">Synopsis</a></li>
<li><a href="#Description">Description</a>
<ul>
<li><a href="#TheDatabase">The Database</a></li>
<li><a href="#remote">The remote API</a></li>
<li><a href="#Zone">Zone</a>
<ul>
<li><a href="#Infrastructure">Infrastructure</a></li>
<li><a href="#ZoneContinued">Zone continued</a></li>
</ul>
</li>
<li><a href="#flightexample1">Flight Example</a></li>
<li><a href="#utc_clock">utc_clock</a></li>
<li><a href="#flightexample2">Flight Example with leap seconds</a></li>
<li><a href="#Formatting">Formatting</a></li>
<li><a href="#Parsing">Parsing</a></li>
<li><a href="#ThreadSafety">Thread Safety</a></li>
</ul>
</li>
<li><a href="#Installation">Installation — this is where you find the implementation</a></li>
<li><a href="#Acknowledgements">Acknowledgements</a></li>
</ul>
<a name="Introduction"></a><h2>Introduction</h2>
<p>
I had just completed writing <a href="date_v2.html"><code>date</code></a>, which is a
library for extending <code><chrono></code> into the realm of calendars, and I was
looking around for the most challenging date time problem I could find with which I could
demonstrate the power of this new library. "I know," I said to myself, "I'll handle all
the world's time zones, and maybe even leap seconds!" Thus began my journey into a
rabbit hole which I knew existed, but had never truly appreciated the intricacies of.
</p>
<p>
This library adds timezone and leap second support to this <a href="date_v2.html">date</a>
library. This is a separate library from <a href="date_v2.html"><code>date</code></a>
because many clients of <a href="date_v2.html"><code>date</code></a> do not need timezone
nor leap second support, and this support does not come for free (though the cost is quite
reasonable).
</p>
<p>
This library is a <b>complete</b> parser of the <a
href="http://www.iana.org/time-zones">IANA Time Zone Database</a>. This database contains
timezone information that represents the history of local time for many representative
locations around the globe. It is updated periodically to reflect changes made by
political bodies to time zone boundaries, UTC offsets, and daylight-saving rules. The
database also maintains a list of leap seconds from 1972 through the present.
</p>
<p>
The <a href="http://www.iana.org/time-zones">IANA Time Zone Database</a> contains four
specific types of data:
</p>
<ol>
<li><p>
Zone: A geographic location with a human-readable name (e.g. "America/New_York") which
specifies the offset from UTC and an abbreviation for the zone. This data includes
daylight saving rules, if applicable, for the zone. This data is not only the rules
currently in effect for the region, but also includes specifications dating back to at
least 1970, and in most cases dating back to the mid 1800's (when uniform time was
first introduced across regions larger than individual towns and cities).
</p></li>
<li><p>
Rule: A specification for a single daylight-saving rule. This helps implement and
consolidate the specifications of Zones.
</p></li>
<li><p>
Link: This is an alternative name for a Zone.
</p></li>
<li><p>
Leap: The date of the insertion of a leap second.
</p></li>
</ol>
<p>
The library documented herein provides access to <i>all</i> of this data, and offers
efficient and convenient ways to compute with it. And this is all done based on the <a
href="date_v2.html"><code>date</code></a> library, which in turn is based on the C++11/14
<code><chrono></code> library. So once you've learned those fundamental libraries,
the learning curve for this library is greatly eased.
</p>
<a name="Overview"></a><h2>Overview</h2>
<p>
Like <a href="date_v2.html"><code>date</code></a>, this library revolves around
<code>std::chrono::system_clock</code>. Thus it is important to understand the properties
of this foundation. <code>std::chrono::system_clock</code> has nested types including
<code>time_point</code> and <code>duration</code>, and a static function called
<code>now</code> which returns a <code>time_point</code>. This <code>time_point</code>
measures time against some unspecified epoch using the units of <code>duration</code>.
</p>
<blockquote><pre>
namespace std { namespace chrono {
class system_clock
{
public:
using duration = microseconds;
using time_point = chrono::time_point<system_clock>;
// other types ...
static time_point now() noexcept;
// other member functions ...
};
}} // namespace std::chrono
</pre></blockquote>
<p>
The use of <code>microseconds</code> above for the <code>duration</code> type is just
an example. This is what is used for <a href="http://libcxx.llvm.org">libc++</a> on
<a href="http://www.apple.com/osx/">OS X</a>. Other implementations may use
other units such as <code>nanoseconds</code>. But one unspecified property that all
implementations of <code>std::chrono::system_clock</code> have in common is that they
all model <a href="https://en.wikipedia.org/wiki/Unix_time">Unix time</a>.
</p>
<p>
<a href="https://en.wikipedia.org/wiki/Unix_time">Unix time</a> counts the number of
seconds since 1970-01-01 00:00:00 UTC, except that leap seconds are ignored in the count.
An interesting and useful property of
<a href="https://en.wikipedia.org/wiki/Unix_time">Unix time</a> is that it treats leap
seconds as nothing more than a typical clock correction. That is, no clock keeps perfect
time, not even the one in your computer. And several times a day your computer will ask
another computer what time it is, and typically correct itself by a small fraction of a
second.
</p>
<p>
When a leap second occurs, instead of counting an extra second,
<a href="https://en.wikipedia.org/wiki/Unix_time">Unix time</a> reacts by saying, oh, I'm
off by a second, I'll slow down or speed up to correct. Companies such as Google will
"smear" the application of a leap second over a period of hours, letting their
<a href="https://en.wikipedia.org/wiki/Unix_time">Unix time</a> counters adjust by
milliseconds, or even microseconds at a time, so that the insertion of a leap second is
virtually undetectable to applications running on the computer.
</p>
<p>
It is this de-facto standard based on
<a href="https://en.wikipedia.org/wiki/Unix_time">Unix time</a> that allows
<a href="date_v2.html"><code>date</code></a> to portably convert a count of microseconds
(for example) into field-based structures containing <code>year/month/day</code> and
<code>hours::minutes::seconds.microseconds</code>. And when you need to further convert
the <code>system_clock::time_point</code> into a local time, and/or take leap seconds into
account, the library documented herein handles it by accessing your local copy of the
<a href="http://www.iana.org/time-zones">IANA Time Zone Database</a>.
</p>
<a name="Synopsis"></a><h2>Synopsis</h2>
<p>
This synopsis provides nothing but a quick overview of the documented API of this library.
If you don't see it in the synopsis below, it is not a documented part of this library.
</p>
<blockquote><pre>
namespace date
{
using second_point = std::chrono::time_point<std::chrono::system_clock,
std::chrono::seconds>;
struct Info
{
second_point begin;
second_point end;
std::chrono::seconds offset;
std::chrono::minutes save;
std::string abbrev;
};
std::ostream&
operator<<(std::ostream& os, const Info& r);
enum class tz {utc, local};
enum class choose {earliest, latest};
class nonexistent_local_time
: public std::runtime_error
{
public:
};
class ambiguous_local_time
: public std::runtime_error
{
public:
};
class Zone
{
public:
const std::string& name() const;
template <class Rep, class Period>
std::pair
<
std::chrono::time_point<std::chrono::system_clock,
typename std::common_type<std::chrono::duration<Rep, Period>,
std::chrono::seconds>::type>,
std::string
>
to_local(std::chrono::time_point<std::chrono::system_clock,
std::chrono::duration<Rep, Period>> tp) const;
template <class Rep, class Period>
std::chrono::time_point<std::chrono::system_clock,
typename std::common_type<std::chrono::duration<Rep, Period>,
std::chrono::seconds>::type>
to_sys(std::chrono::time_point<std::chrono::system_clock,
std::chrono::duration<Rep, Period>> tp) const;
template <class Rep, class Period>
std::chrono::time_point<std::chrono::system_clock,
typename std::common_type<std::chrono::duration<Rep, Period>,
std::chrono::seconds>::type>
to_sys(std::chrono::time_point<std::chrono::system_clock,
std::chrono::duration<Rep, Period>> tp,
choose z) const;
template <class Rep, class Period>
Info
get_info(std::chrono::time_point<std::chrono::system_clock,
std::chrono::duration<Rep, Period>> tp,
tz timezone) const;
};
const Zone* locate_zone(const std::string& tz_name);
const Zone* current_zone();
bool operator==(const Zone& x, const Zone& y);
bool operator!=(const Zone& x, const Zone& y);
bool operator< (const Zone& x, const Zone& y);
bool operator> (const Zone& x, const Zone& y);
bool operator<=(const Zone& x, const Zone& y);
bool operator>=(const Zone& x, const Zone& y);
std::ostream& operator<<(std::ostream& os, const Zone& z);
class Link
{
public:
const std::string& name() const;
const std::string& target() const;
};
bool operator==(const Link& x, const Link& y);
bool operator!=(const Link& x, const Link& y);
bool operator< (const Link& x, const Link& y);
bool operator> (const Link& x, const Link& y);
bool operator<=(const Link& x, const Link& y);
bool operator>=(const Link& x, const Link& y);
std::ostream& operator<<(std::ostream& os, const Link& x);
class Leap
{
public:
second_point date() const;
};
bool operator==(const Leap& x, const Leap& y);
bool operator!=(const Leap& x, const Leap& y);
bool operator< (const Leap& x, const Leap& y);
bool operator> (const Leap& x, const Leap& y);
bool operator<=(const Leap& x, const Leap& y);
bool operator>=(const Leap& x, const Leap& y);
std::ostream& operator<<(std::ostream& os, const Leap& x);
template <class Duration>
bool
operator==(const Leap& x,
const std::chrono::time_point<std::chrono::system_clock, Duration>& y);
template <class Duration>
bool
operator==(const std::chrono::time_point<std::chrono::system_clock, Duration>& x,
const Leap& y);
template <class Duration>
bool
operator!=(const Leap& x,
const std::chrono::time_point<std::chrono::system_clock, Duration>& y);
template <class Duration>
bool
operator!=(const std::chrono::time_point<std::chrono::system_clock, Duration>& x,
const Leap& y);
template <class Duration>
bool
operator< (const Leap& x,
const std::chrono::time_point<std::chrono::system_clock, Duration>& y);
template <class Duration>
bool
operator< (const std::chrono::time_point<std::chrono::system_clock, Duration>& x,
const Leap& y);
template <class Duration>
bool
operator> (const Leap& x,
const std::chrono::time_point<std::chrono::system_clock, Duration>& y);
template <class Duration>
bool
operator> (const std::chrono::time_point<std::chrono::system_clock, Duration>& x,
const Leap& y);
template <class Duration>
bool
operator<=(const Leap& x,
const std::chrono::time_point<std::chrono::system_clock, Duration>& y);
template <class Duration>
bool
operator<=(const std::chrono::time_point<std::chrono::system_clock, Duration>& x,
const Leap& y);
template <class Duration>
bool
operator>=(const Leap& x,
const std::chrono::time_point<std::chrono::system_clock, Duration>& y);
template <class Duration>
bool
operator>=(const std::chrono::time_point<std::chrono::system_clock, Duration>& x,
const Leap& y);
class Rule;
struct TZ_DB
{
std::string version;
std::vector<Zone> zones;
std::vector<Link> links;
std::vector<Leap> leaps;
std::vector<Rule> rules;
};
std::ostream& operator<<(std::ostream& os, const TZ_DB& db);
const TZ_DB& get_tzdb();
const TZ_DB& reload_tzdb();
#if HAS_REMOTE_API
std::string remote_version();
bool remote_download(const std::string& version);
bool remote_install(const std::string& version);
#endif
class utc_clock
{
public:
using duration = std::chrono::system_clock::duration;
using rep = duration::rep;
using period = duration::period;
using time_point = std::chrono::time_point<utc_clock>;
static constexpr bool is_steady = true;
static time_point now() noexcept;
template <class Duration>
static
std::chrono::time_point<utc_clock,
typename std::common_type<Duration, std::chrono::seconds>::type>
sys_to_utc(std::chrono::time_point<std::chrono::system_clock, Duration> t);
template <class Duration>
static
std::chrono::time_point<std::chrono::system_clock,
typename std::common_type<Duration, std::chrono::seconds>::type>
utc_to_sys(std::chrono::time_point<utc_clock, Duration> t);
};
template <class Duration>
std::string
format(const std::locale& loc, std::string format,
std::chrono::time_point<std::chrono::system_clock, Duration> tp,
const Zone* zone = nullptr);
std::string
format(const std::locale& loc, std::string format, day_point tp,
const Zone* zone = nullptr);
template <class Duration>
std::string
format(std::string format,
std::chrono::time_point<std::chrono::system_clock, Duration> tp,
const Zone* zone = nullptr);
std::string
format(std::string format, day_point tp, const Zone* zone = nullptr);
template <class Duration>
void
parse(std::istream& is, const std::string& format,
std::chrono::time_point<std::chrono::system_clock, Duration>& tp);
template <class Duration>
void
parse(std::istream& is, const std::string& format,
std::chrono::time_point<std::chrono::system_clock, Duration>& tp,
std::string& abbrev);
} // namespace date
</pre></blockquote>
<a name="Description"></a><h2>Description</h2>
<p>
Everything documented below is in <code>namespace date</code>. Explicit references to
this namespace in example code below is intentionally omitted in the hopes of reducing
verbosity.
</p>
<a name="TheDatabase"></a><h3>The Database</h3>
<p>
The database is represented with the type <code>TZ_DB</code>:
</p>
<blockquote><pre>
struct TZ_DB
{
std::string version;
std::vector<Zone> zones;
std::vector<Link> links;
std::vector<Leap> leaps;
std::vector<Rule> rules;
};
</pre></blockquote>
<p>
This is a singleton class. You can get a <code>const TZ_DB&</code> to the singleton
using this function:
</p>
<blockquote><pre>
const TZ_DB& get_tzdb();
</pre></blockquote>
<p>
The first call to <code>get_tzdb()</code> will initialize the database from your local
copy of the <a href="http://www.iana.org/time-zones">IANA Time Zone Database</a> located
at <code>install</code> (a file-scope variable of type <code>std::string</code> in
<a href="https://github.com/HowardHinnant/date/blob/master/tz.cpp"><code>tz.cpp</code></a>).
You will need to catch the return of this function by <code>const&</code> as the
<code>TZ_DB</code> is not constructible from a <code>const TZ_DB</code>. This can be done
with the following example code:
</p>
<blockquote><pre>
auto& db = get_tzdb();
</pre></blockquote>
<p>
With a reference to the database in hand, you have read-only access to the entire
database, which is nothing more than sorted <code>vector</code>s for the four types of
data contained in the database. With such a reference you could (for example) print the
names of all the Zones in the database:
</p>
<blockquote><pre>
for (auto& z : db.zones)
std::cout << z.name() << '\n';
</pre></blockquote>
<p>
There are currently 377 zones in the database.
</p>
<p>
Or you could output the 89 <code>Link</code>s, including their <code>name()</code> and
<code>target()</code>:
</p>
<blockquote><pre>
for (auto& link : db.links)
std::cout << link << '\n';
</pre></blockquote>
<p>
If you aren't happy with the format this outputs in, <code>Link</code> has public member
functions <code>name()</code> and <code>target()</code> so that you can achieve whatever
format you desire.
</p>
<p>
If needed, <code>db.version</code> is a <code>std::string</code> containing the
<a href="http://www.iana.org/time-zones">IANA Time Zone Database</a> version of the
database you are reading. For example the current version when this sentence
was written was "2016a".
</p>
<p>
You can even print the entire database out in a semi-human-readable format if desired:
</p>
<blockquote><pre>
std::cout << db << '\n';
</pre></blockquote>
<p>
If you constrain the geography or history of the database during installation, those
constraints will be reflected in these examples.
</p>
<p>
If you decide you need to reload the database say, because you want to install a new
version of the <a href="http://www.iana.org/time-zones">IANA Time Zone Database</a>
without stopping your program, you can use this function:
</p>
<blockquote><pre>
const TZ_DB& reload_tzdb();
</pre></blockquote>
<p>
This re-initializes the database by reading from the <code>install</code> location you
customized on installation. The use of the <code>reload_tzdb</code> function is not
pain-free, and not for every application (not for most of them I'm guessing). For example
see the <b>Thread Safety</b> section for issues related to the use of these functions.
</p>
<a name="remote"></a><h3>The remote API</h3>
<p>
The remote API is enabled only if <code>HAS_REMOTE_API</code> is set to 1 during
compilation. See <a href="#Installation">Installation</a> for more details.
</p>
<blockquote><pre>
std::string remote_version();
</pre></blockquote>
<p>
This function will query the
<a href="http://www.iana.org/time-zones">IANA Time Zone Database website</a> for the
latest version number of the IANA database, and return it as a <code>std::string</code>.
If an internet connection can not be made, an empty <code>string</code> is returned.
This string can be compared against the version of your local copy of the database:
<code>get_tzdb().version</code>.
</p>
<blockquote><pre>
bool remote_download(const std::string& version);
</pre></blockquote>
<p>
This function will attempt to download the database with the version <code>version</code>
from the <a href="http://www.iana.org/time-zones">IANA Time Zone Database website</a>.
If successful, <code>true</code> is returned and a file named
<code>version + ".tar.gz"</code> will be stored at the location <code>install</code>.
If not successful, <code>false</code> is returned.
</p>
<blockquote><pre>
bool remote_install(const std::string& version);
</pre></blockquote>
<p>
This function will attempt to uncompress the tar file downloaded by
<code>remote_download(version)</code> and replace any existing database with
the result. It will then delete the tar file. If the tar file doesn't exist,
<code>remote_install</code> will do nothing. Returns <code>true</code> on
success, else returns <code>false</code>.
</p>
<a name="Zone"></a><h3>Zone</h3>
<p>
The <code>Zone</code> class is the most important type in this library. It provides the
main access to the functionality provided by this library. Each <code>Zone</code> is
named, represents a geographic area, and provides a mapping between UTC and the local
time, in both directions. This mapping from local time to UTC is in general not one to
one. The mapping, and even the specific rule, depends upon the input
<code>time_point</code>, which can represent either UTC or local time.
</p>
<p>
The detailed API of the <code>Zone</code> class depends upon a small amount of
infrastructure which is introduced first.
</p>
<a name="Infrastructure"></a><h4>Infrastructure</h4>
<blockquote><pre>
using second_point = std::chrono::time_point<std::chrono::system_clock,
std::chrono::seconds>;
</pre></blockquote>
<p>
<code>second_point</code> is a <code>std::chrono::time_point</code> based on
<code>system_clock</code> but with the precision of <code>seconds</code>. This library
will interoperate with <code>system_clock::time_point</code>s of <i>any</i> precision.
However the data in the database is largely based on <code>second_point</code>, and
some of the data which is presented, such as that in the <code>Info</code> class, uses
this type alias as a convenience, and to reduce verbosity. <code>second_point</code>
will implicitly convert to <code>system_clock::time_point</code>. And coarser
<code>time_point</code>s such as the <code>day_point</code> from the
<a href="date_v2.html"><code>date</code></a> library will implicitly convert to
<code>second_point</code>.
</p>
<blockquote><pre>
struct Info
{
second_point begin;
second_point end;
std::chrono::seconds offset;
std::chrono::minutes save;
std::string abbrev;
};
</pre></blockquote>
<p>
The <code>Info</code> struct is the return type of the <code>get_info</code> member
function of the <code>Zone</code> class. It contains very detailed information about the
<code>Zone</code> at the <code>time_point</code> (UTC or local) input into this member
function. <code>Info</code> contains no pointers or references into the database.
Therefore clients do not need to be concerned about holding on to <code>Info</code>s
during a call to <code>reload_tzdb()</code>. Though a call to <code>reload_tzdb()</code>
could potentially make the data in an outstanding <code>Info</code> obsolete. See
<code>Zone::get_info</code> for more details.
</p>
<blockquote><pre>
enum class tz {utc, local};
enum class choose {earliest, latest};
</pre></blockquote>
<p>
These <code>enum</code>s are used as input to some of the <code>Zone</code> member
functions. <code>tz::utc</code> indicates that a <code>time_point</code> represents a
time in the UTC time zone. <code>tz::local</code> indicates that a
<code>time_point</code> represents a time in the <code>Zone</code>'s local time zone.
The <code>choose enum</code> allows a client to specify how a mapping from local to UTC
should behave when the mapping is not one to one. Alternatively one can not specify
a policy in the mapping, and if the mapping is not unique, an exception will be thrown.
</p>
<blockquote><pre>
class nonexistent_local_time
: public std::runtime_error
{
public:
const char* what() const override;
};
class ambiguous_local_time
: public std::runtime_error
{
public:
const char* what() const override;
};
</pre></blockquote>
<p>
These are the exception classes thrown by the local to UTC mapping. In addition to their
type indicating the nature of the exceptional circumstance, they also sport a
<code>what()</code> member function that will contain a very detailed explanation
including specific times for the specific <code>time_point</code>s involved in the
attempted mapping.
</p>
<p>
If in a call to <code>Zone::to_sys</code> the local <code>time_point</code> falls into a
"gap" for which no local time exists, a <code>nonexistent_local_time</code> exception is
thrown.
</p>
<p>
If in a call to <code>Zone::to_sys</code> the local <code>time_point</code> has an
ambiguous mapping to UTC, a <code>ambiguous_local_time</code> exception is thrown.
</p>
<p>
Either exceptional situation can be circumvented with the use of
<code>choose::earliest</code> or <code>choose::latest</code> in the call to
<code>to_sys</code>.
</p>
<a name="ZoneContinued"></a><h4>Zone continued</h4>
<blockquote><pre>
class Zone
{
public:
const std::string& name() const;
template <class Rep, class Period>
std::pair
<
std::chrono::time_point<std::chrono::system_clock,
typename std::common_type<std::chrono::duration<Rep, Period>,
std::chrono::seconds>::type>,
std::string
>
to_local(std::chrono::time_point<std::chrono::system_clock,
std::chrono::duration<Rep, Period>> tp) const;
template <class Rep, class Period>
std::chrono::time_point<std::chrono::system_clock,
typename std::common_type<std::chrono::duration<Rep, Period>,
std::chrono::seconds>::type>
to_sys(std::chrono::time_point<std::chrono::system_clock,
std::chrono::duration<Rep, Period>> tp) const;
template <class Rep, class Period>
std::chrono::time_point<std::chrono::system_clock,
typename std::common_type<std::chrono::duration<Rep, Period>,
std::chrono::seconds>::type>
to_sys(std::chrono::time_point<std::chrono::system_clock,
std::chrono::duration<Rep, Period>> tp,
choose z) const;
template <class Rep, class Period>
Info
get_info(std::chrono::time_point<std::chrono::system_clock,
std::chrono::duration<Rep, Period>> tp,
tz timezone) const;
};
const Zone* locate_zone(const std::string& tz_name);
const Zone* current_zone();
bool operator==(const Zone& x, const Zone& y);
bool operator!=(const Zone& x, const Zone& y);
bool operator< (const Zone& x, const Zone& y);
bool operator> (const Zone& x, const Zone& y);
bool operator<=(const Zone& x, const Zone& y);
bool operator>=(const Zone& x, const Zone& y);
std::ostream& operator<<(std::ostream& os, const Zone& z);
</pre></blockquote>
<p>
The entire public API of the <code>Zone</code> is <code>const</code>. Once the database
is initialized (or reloaded), <code>Zone</code>s are set in concrete.
</p>
<hr>
<p>
The current time zone associated with your computer can be retrieved with the namespace
scope function <code>current_zone()</code>. For example:
</p>
<blockquote><pre>
std::cout << current_zone()->name() << '\n';
</pre></blockquote>
<p>
For me the above currently outputs <code>America/New_York</code>.
</p>
<hr>
<blockquote><pre>
const Zone* locate_zone(const std::string& tz_name);
</pre></blockquote>
<p>
<code>locate_zone</code> returns a pointer to a <code>Zone</code> in the database
associated with <code>tz_name</code>. If it can't find a <code>Zone</code> named
<code>tz_name</code>, the implementation will search for a <code>Link</code> named
<code>tz_name</code>, and then return the <code>Zone</code> associated with the
<code>Link</code>'s <code>target()</code>. If <code>tz_name</code> can not be found in the
database, a <code>std::runtime_error</code> is thrown.
</p>
<p>
Example:
</p>
<blockquote><pre>
try
{
cout << locate_zone("Europe/London")->name() << '\n'; // A Zone
cout << locate_zone("Europe/Jersey")->name() << '\n'; // A Link to a Zone
cout << locate_zone("Europe/New_Jersey")->name() << '\n'; // Doesn't exist
}
catch (const exception& e)
{
cout << e.what() << '\n';
}
</pre></blockquote>
<p>
Which outputs:
</p>
<blockquote><pre>
Europe/London
Europe/London
Europe/New_Jersey not found in timezone database
</pre></blockquote>
<p>
Note that <code>locate_zone</code> never returns <code>nullptr</code>. Also note that
the first call to <code>locate_zone</code> may implicitly initialize the database.
</p>
<hr>
<blockquote><pre>
template <class Rep, class Period>
std::pair
<
std::chrono::time_point<std::chrono::system_clock,
typename std::common_type<std::chrono::duration<Rep, Period>,
std::chrono::seconds>::type>,
std::string
>
to_local(std::chrono::time_point<std::chrono::system_clock,
std::chrono::duration<Rep, Period>> tp) const;
</pre></blockquote>
<p>
<code>to_local</code> maps a <code>system_clock</code>-associated <code>time_point</code>
from UTC to local time, returning both the mapped <code>time_point</code> and an
abbreviation for the local time zone. This member function accepts any precision
<code>time_point</code>, but returns a <code>time_point</code> with a precision of
<code>seconds</code> or finer. This is done because it is possible that some of the
mappings returned by the database need the precision of a second.
</p>
<p>
There are only two ways this function can fail:
</p>
<ol>
<li><p>
Out of memory error. Not bloody likely. The only memory that possibly could be
allocated is for the abbreviation stored in a <code>std::string</code> and all known
implementations will fit all known abbreviations into their short string buffer.
</p></li>
<li><p>
If you curtailed history during installation, a <code>runtime_error</code> will be thrown
if <code>tp</code> refers to a <code>time_point</code> outside of the range
<code>min_year/jan/1 00:00:00</code> to <code>max_year/dec/31 23:59:59</code>. This can
not happen with the default settings of <code>min_year</code> and <code>max_year</code>.
</p></li>
</ol>
<p>
Example:
</p>
<blockquote><pre>
auto local = current_zone()->to_local(system_clock::now());
cout << local.first << ' ' << local.second << '\n';
</pre></blockquote>
<p>
Which just output for me:
</p>
<blockquote><pre>
2015-07-12 16:57:14.430467 EDT
</pre></blockquote>
<p>
Not quite 5pm in the US Eastern timezone during daylight saving time.
</p>
<p>
And for a historical example:
</p>
<blockquote><pre>
auto distant_past = locate_zone("America/New_York")->to_local(day_point(feb/9/1942) + 7h);
cout << distant_past.first << ' ' << distant_past.second << '\n';
</pre></blockquote>
<p>
Which outputs:
</p>
<blockquote><pre>
1942-02-09 03:00:00 EWT
</pre></blockquote>
<p>
The US shifted to "War Time."
</p>
<hr>
<p>
If you want to go the other direction (from local time to UTC) use:
</p>
<blockquote><pre>
template <class Rep, class Period>
std::chrono::time_point<std::chrono::system_clock,
typename std::common_type<std::chrono::duration<Rep, Period>,
std::chrono::seconds>::type>
to_sys(std::chrono::time_point<std::chrono::system_clock,
std::chrono::duration<Rep, Period>> tp) const;
</pre></blockquote>
<p>
For example:
</p>
<blockquote><pre>
auto distant_past = locate_zone("America/New_York")->to_sys(day_point(feb/9/1942) + 3h);
cout << distant_past << ' ' << " UTC\n";
</pre></blockquote>
<p>
Which outputs:
</p>
<blockquote><pre>
1942-02-09 07:00:00 UTC
</pre></blockquote>
<p>
This function will throw an exception of type <code>nonexistent_local_time</code> if the
local time does not exist. This can happen when the local clock is discontinuously set
forward, such as when moving from standard time to daylight savings time.
</p>
<p>
For example:
</p>
<blockquote><pre>
try
{
auto distant_past = locate_zone("America/New_York")->to_sys(day_point(feb/9/1942) + 3h - 1ms);
cout << distant_past << ' ' << " UTC\n";
}
catch (const exception& e)
{
cout << e.what() << '\n';
}
</pre></blockquote>
<p>