[time.clock.utc]

# 30 Time library [[time]](./#time)

## 30.7 Clocks [[time.clock]](time.clock#utc)

### 30.7.3 Class utc_clock [time.clock.utc]

#### [30.7.3.1](#overview) Overview [[time.clock.utc.overview]](time.clock.utc.overview)

namespace std::chrono {class utc_clock {public:using rep = *a signed arithmetic type*; using period = ratio<*unspecified*, *unspecified*>; using duration = chrono::duration<rep, period>; using time_point = chrono::time_point<utc_clock>; static constexpr bool is_steady = *unspecified*; static time_point now(); template<class Duration>static sys_time<common_type_t<Duration, seconds>> to_sys(const utc_time<Duration>& t); template<class Duration>static utc_time<common_type_t<Duration, seconds>> from_sys(const sys_time<Duration>& t); };}

[1](#overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2834)

In contrast to sys_time,
which does not take leap seconds into account,utc_clock and its associated time_point, utc_time,
count time, including leap seconds, since 1970-01-01 00:00:00 UTC.

[*Note [1](#overview-note-1)*:

The UTC time standard began on 1972-01-01 00:00:10 TAI.
To measure time since this epoch instead, one can add/subtract the constantsys_days{1972y/1/1} - sys_days{1970y/1/1} (63'072'000s)
from the utc_time[.](#overview-1.sentence-1)

— *end note*]

[*Example [1](#overview-example-1)*:   


clock_cast<utc_clock>(sys_seconds{sys_days{1970y/January/1}}).time_since_epoch() is 0s[.](#overview-1.sentence-2)

  


clock_cast<utc_clock>(sys_seconds{sys_days{2000y/January/1}}).time_since_epoch() is 946'684'822s,  

which is 10'957 * 86'400s + 22s[.](#overview-1.sentence-4)

  
 — *end example*]

[2](#overview-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2852)

utc_clock is not a [*Cpp17TrivialClock*](time.clock.req#:Cpp17TrivialClock "30.3 Cpp17Clock requirements [time.clock.req]") unless the implementation can guarantee that utc_clock​::​now() does not propagate an exception[.](#overview-2.sentence-1)

[*Note [2](#overview-note-2)*:

noexcept(from_sys(system_clock​::​now())) is false[.](#overview-2.sentence-2)

— *end note*]

#### [30.7.3.2](#members) Member functions [[time.clock.utc.members]](time.clock.utc.members)

[🔗](#lib:now,utc_clock)

`static time_point now();
`

[1](#members-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2868)

*Returns*: from_sys(system_clock​::​now()), or a more accurate value of utc_time[.](#members-1.sentence-1)

[🔗](#lib:to_sys,utc_clock)

`template<class Duration>
  static sys_time<common_type_t<Duration, seconds>>
    to_sys(const utc_time<Duration>& u);
`

[2](#members-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2881)

*Returns*: A sys_time t,
such that from_sys(t) == u if such a mapping exists[.](#members-2.sentence-1)

Otherwise u represents a time_point during a positive leap second insertion,
the conversion counts that leap second as not inserted,
and the last representable value of sys_time prior to the insertion of the leap second is returned[.](#members-2.sentence-2)

[🔗](#lib:from_sys,utc_clock)

`template<class Duration>
  static utc_time<common_type_t<Duration, seconds>>
    from_sys(const sys_time<Duration>& t);
`

[3](#members-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2900)

*Returns*: A utc_time u, such thatu.time_since_epoch() - t.time_since_epoch() is equal to the sum of leap seconds that were inserted
between t and 1970-01-01[.](#members-3.sentence-1)

If t is exactly the date of leap second insertion,
then the conversion counts that leap second as inserted[.](#members-3.sentence-2)

[*Example [1](#members-example-1)*: auto t = sys_days{July/1/2015} - 2ns;auto u = utc_clock::from_sys(t);
assert(u.time_since_epoch() - t.time_since_epoch() == 25s);
t += 1ns;
u = utc_clock::from_sys(t);
assert(u.time_since_epoch() - t.time_since_epoch() == 25s);
t += 1ns;
u = utc_clock::from_sys(t);
assert(u.time_since_epoch() - t.time_since_epoch() == 26s);
t += 1ns;
u = utc_clock::from_sys(t);
assert(u.time_since_epoch() - t.time_since_epoch() == 26s); — *end example*]

#### [30.7.3.3](#nonmembers) Non-member functions [[time.clock.utc.nonmembers]](time.clock.utc.nonmembers)

[🔗](#lib:operator%3c%3c,utc_time)

`template<class charT, class traits, class Duration>
  basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const utc_time<Duration>& t);
`

[1](#nonmembers-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2937)

*Effects*: Equivalent to:return os << format(os.getloc(), *STATICALLY-WIDEN*<charT>("{:L%F %T}"), t);

[2](#nonmembers-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2944)

[*Example [1](#nonmembers-example-1)*: auto t = sys_days{July/1/2015} - 500ms;auto u = clock_cast<utc_clock>(t);for (auto i = 0; i < 8; ++i, u += 250ms) cout << u << " UTC\n";

Produces this output:

```
2015-06-30 23:59:59.500 UTC
2015-06-30 23:59:59.750 UTC
2015-06-30 23:59:60.000 UTC
2015-06-30 23:59:60.250 UTC
2015-06-30 23:59:60.500 UTC
2015-06-30 23:59:60.750 UTC
2015-07-01 00:00:00.000 UTC
2015-07-01 00:00:00.250 UTC

```

— *end example*]

[🔗](#lib:from_stream,utc_time)

`template<class charT, class traits, class Duration, class Alloc = allocator<charT>>
  basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                utc_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);
`

[3](#nonmembers-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2977)

*Effects*: Attempts to parse the input stream is into the utc_time tp using
the format flags given in the NTCTS fmt as specified in [[time.parse]](time.parse "30.13 Parsing")[.](#nonmembers-3.sentence-1)

If the parse fails to decode a valid date,is.setstate(ios_base​::​failbit) is called andtp is not modified[.](#nonmembers-3.sentence-2)

If %Z is used and successfully parsed,
that value will be assigned to *abbrev if abbrev is non-null[.](#nonmembers-3.sentence-3)

If %z (or a modified variant) is used and successfully parsed,
that value will be assigned to *offset if offset is non-null[.](#nonmembers-3.sentence-4)

Additionally, the parsed offset will be subtracted from
the successfully parsed timestamp
prior to assigning that difference to tp[.](#nonmembers-3.sentence-5)

[4](#nonmembers-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2994)

*Returns*: is[.](#nonmembers-4.sentence-1)

[🔗](#lib:leap_second_info)

`struct leap_second_info {
  bool    is_leap_second;
  seconds elapsed;
};
`

[5](#nonmembers-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3008)

The type leap_second_info has data members and special members specified above[.](#nonmembers-5.sentence-1)

It has no base classes or members other than those specified[.](#nonmembers-5.sentence-2)

[🔗](#lib:get_leap_second_info)

`template<class Duration>
  leap_second_info get_leap_second_info(const utc_time<Duration>& ut);
`

[6](#nonmembers-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3021)

*Returns*: A leap_second_info lsi,
where lsi.is_leap_second is true if ut is during a positive leap second insertion, and
otherwise false[.](#nonmembers-6.sentence-1)

lsi.elapsed is the sum of leap seconds between 1970-01-01 and ut[.](#nonmembers-6.sentence-2)

If lsi.is_leap_second is true,
the leap second referred to by ut is included in the sum[.](#nonmembers-6.sentence-3)