[time.clock.cast]

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

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

### 30.7.10 time_point conversions [time.clock.cast]

#### [30.7.10.1](#time.clock.conv) Class template clock_time_conversion [[time.clock.conv]](time.clock.conv)

namespace std::chrono {template<class DestClock, class SourceClock>struct clock_time_conversion {};}

[1](#time.clock.conv-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3583)

clock_time_conversion serves as a trait
which can be used to specify how to convert
a source time_point of typetime_point<SourceClock, Duration> to a destination time_point of typetime_point<DestClock, Duration> via a specialization:clock_time_conversion<DestClock, SourceClock>[.](#time.clock.conv-1.sentence-1)

A specialization of clock_time_conversion<DestClock, SourceClock> shall provide a const-qualified operator() that takes a parameter of type time_point<SourceClock, Duration> and returns a time_point<DestClock, OtherDuration> representing an equivalent point in time[.](#time.clock.conv-1.sentence-2)

OtherDuration is a chrono​::​duration whose specialization is computed from the input Duration in a manner which can vary for each clock_time_conversion specialization[.](#time.clock.conv-1.sentence-3)

A program may specialize clock_time_conversion if at least one of the template parameters is a user-defined clock type[.](#time.clock.conv-1.sentence-4)

[2](#time.clock.conv-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3603)

Several specializations are provided by the implementation,
as described in[[time.clock.cast.id]](#id "30.7.10.2 Identity conversions"),[[time.clock.cast.sys.utc]](#sys.utc "30.7.10.3 Conversions between system_­clock and utc_­clock"),[[time.clock.cast.sys]](#sys "30.7.10.4 Conversions between system_­clock and other clocks"), and[[time.clock.cast.utc]](#utc "30.7.10.5 Conversions between utc_­clock and other clocks")[.](#time.clock.conv-2.sentence-1)

#### [30.7.10.2](#id) Identity conversions [[time.clock.cast.id]](time.clock.cast.id)

template<class Clock>struct clock_time_conversion<Clock, Clock> {template<class Duration> time_point<Clock, Duration>operator()(const time_point<Clock, Duration>& t) const;};

[🔗](#lib:operator(),clock_time_conversion)

`template<class Duration>
  time_point<Clock, Duration>
    operator()(const time_point<Clock, Duration>& t) const;
`

[1](#id-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3630)

*Returns*: t[.](#id-1.sentence-1)

template<>struct clock_time_conversion<system_clock, system_clock> {template<class Duration> sys_time<Duration>operator()(const sys_time<Duration>& t) const;};

[🔗](#lib:operator(),clock_time_conversion_)

`template<class Duration>
  sys_time<Duration>
    operator()(const sys_time<Duration>& t) const;
`

[2](#id-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3652)

*Returns*: t[.](#id-2.sentence-1)

template<>struct clock_time_conversion<utc_clock, utc_clock> {template<class Duration> utc_time<Duration>operator()(const utc_time<Duration>& t) const;};

[🔗](#lib:operator(),clock_time_conversion__)

`template<class Duration>
  utc_time<Duration>
    operator()(const utc_time<Duration>& t) const;
`

[3](#id-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3674)

*Returns*: t[.](#id-3.sentence-1)

#### [30.7.10.3](#sys.utc) Conversions between system_clock and utc_clock [[time.clock.cast.sys.utc]](time.clock.cast.sys.utc)

template<>struct clock_time_conversion<utc_clock, system_clock> {template<class Duration> utc_time<common_type_t<Duration, seconds>>operator()(const sys_time<Duration>& t) const;};

[🔗](#lib:operator(),clock_time_conversion___)

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

[1](#sys.utc-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3698)

*Returns*: utc_clock​::​from_sys(t)[.](#sys.utc-1.sentence-1)

template<>struct clock_time_conversion<system_clock, utc_clock> {template<class Duration> sys_time<common_type_t<Duration, seconds>>operator()(const utc_time<Duration>& t) const;};

[🔗](#lib:operator(),clock_time_conversion____)

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

[2](#sys.utc-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3720)

*Returns*: utc_clock​::​to_sys(t)[.](#sys.utc-2.sentence-1)

#### [30.7.10.4](#sys) Conversions between system_clock and other clocks [[time.clock.cast.sys]](time.clock.cast.sys)

template<class SourceClock>struct clock_time_conversion<system_clock, SourceClock> {template<class Duration>auto operator()(const time_point<SourceClock, Duration>& t) const-> decltype(SourceClock::to_sys(t));};

[🔗](#lib:operator(),clock_time_conversion_____)

`template<class Duration>
  auto operator()(const time_point<SourceClock, Duration>& t) const
    -> decltype(SourceClock::to_sys(t));
`

[1](#sys-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3744)

*Constraints*: SourceClock​::​to_sys(t) is well-formed[.](#sys-1.sentence-1)

[2](#sys-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3748)

*Mandates*: SourceClock​::​to_sys(t) returns a sys_time<Duration2> for some type Duration2 ([[time.point.general]](time.point.general "30.6.1 General"))[.](#sys-2.sentence-1)

[3](#sys-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3753)

*Returns*: SourceClock​::​to_sys(t)[.](#sys-3.sentence-1)

template<class DestClock>struct clock_time_conversion<DestClock, system_clock> {template<class Duration>auto operator()(const sys_time<Duration>& t) const-> decltype(DestClock::from_sys(t));};

[🔗](#lib:operator(),clock_time_conversion______)

`template<class Duration>
  auto operator()(const sys_time<Duration>& t) const
    -> decltype(DestClock::from_sys(t));
`

[4](#sys-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3775)

*Constraints*: DestClock​::​from_sys(t) is well-formed[.](#sys-4.sentence-1)

[5](#sys-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3779)

*Mandates*: DestClock​::​from_sys(t) returns a time_point<DestClock, Duration2> for some type Duration2 ([[time.point.general]](time.point.general "30.6.1 General"))[.](#sys-5.sentence-1)

[6](#sys-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3784)

*Returns*: DestClock​::​from_sys(t)[.](#sys-6.sentence-1)

#### [30.7.10.5](#utc) Conversions between utc_clock and other clocks [[time.clock.cast.utc]](time.clock.cast.utc)

template<class SourceClock>struct clock_time_conversion<utc_clock, SourceClock> {template<class Duration>auto operator()(const time_point<SourceClock, Duration>& t) const-> decltype(SourceClock::to_utc(t));};

[🔗](#lib:operator(),clock_time_conversion_______)

`template<class Duration>
  auto operator()(const time_point<SourceClock, Duration>& t) const
    -> decltype(SourceClock::to_utc(t));
`

[1](#utc-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3808)

*Constraints*: SourceClock​::​to_utc(t) is well-formed[.](#utc-1.sentence-1)

[2](#utc-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3812)

*Mandates*: SourceClock​::​to_utc(t) returns a utc_time<Duration2> for some type Duration2 ([[time.point.general]](time.point.general "30.6.1 General"))[.](#utc-2.sentence-1)

[3](#utc-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3817)

*Returns*: SourceClock​::​to_utc(t)[.](#utc-3.sentence-1)

template<class DestClock>struct clock_time_conversion<DestClock, utc_clock> {template<class Duration>auto operator()(const utc_time<Duration>& t) const-> decltype(DestClock::from_utc(t));};

[🔗](#lib:operator(),clock_time_conversion________)

`template<class Duration>
  auto operator()(const utc_time<Duration>& t) const
    -> decltype(DestClock::from_utc(t));
`

[4](#utc-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3839)

*Constraints*: DestClock​::​from_utc(t) is well-formed[.](#utc-4.sentence-1)

[5](#utc-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3843)

*Mandates*: DestClock​::​from_utc(t) returns a time_point<DestClock, Duration2> for some type Duration2 ([[time.point.general]](time.point.general "30.6.1 General"))[.](#utc-5.sentence-1)

[6](#utc-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3848)

*Returns*: DestClock​::​from_utc(t)[.](#utc-6.sentence-1)

#### [30.7.10.6](#fn) Function template clock_cast [[time.clock.cast.fn]](time.clock.cast.fn)

[🔗](#lib:clock_cast)

`template<class DestClock, class SourceClock, class Duration>
  auto clock_cast(const time_point<SourceClock, Duration>& t);
`

[1](#fn-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3862)

*Constraints*: At least one of the following clock time conversion expressions
is well-formed:

- [(1.1)](#fn-1.1)

clock_time_conversion<DestClock, SourceClock>{}(t)

- [(1.2)](#fn-1.2)

clock_time_conversion<DestClock, system_clock>{}( clock_time_conversion<system_clock, SourceClock>{}(t))

- [(1.3)](#fn-1.3)

clock_time_conversion<DestClock, utc_clock>{}( clock_time_conversion<utc_clock, SourceClock>{}(t))

- [(1.4)](#fn-1.4)

clock_time_conversion<DestClock, utc_clock>{}( clock_time_conversion<utc_clock, system_clock>{}( clock_time_conversion<system_clock, SourceClock>{}(t)))

- [(1.5)](#fn-1.5)

clock_time_conversion<DestClock, system_clock>{}( clock_time_conversion<system_clock, utc_clock>{}( clock_time_conversion<utc_clock, SourceClock>{}(t)))

A clock time conversion expression is considered better than
another clock time conversion expression if it involves feweroperator() calls on clock_time_conversion specializations[.](#fn-1.sentence-2)

[2](#fn-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3904)

*Mandates*: Among the well-formed clock time conversion expressions
from the above list, there is a unique best expression[.](#fn-2.sentence-1)

[3](#fn-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3909)

*Returns*: The best well-formed clock time conversion expression in the above list[.](#fn-3.sentence-1)