`<chrono>` functions

Article
10/17/2022

`abs(duration)`

Returns d if d >= d.zero(); otherwise returns -d.

Syntax

template <class Rep, class Period>
constexpr duration<Rep, Period> abs(duration<Rep, Period> d ); // C++17

Parameters

Rep
The type of the internal representation of the source duration d.

Period
A std::ratio type representing the ratio of one second to the source Rep type (that is, seconds per Rep).

d
The source duration object.

Return value

The absolute value of d.

Example: `abs(duration)`

// compile using: /std:c++latest
#include <chrono>
#include <iostream>

int main()
{
    std::cout << abs(-24h);
    return 0;
}

24h

`ceil(duration)`

Returns the smallest representable duration in the target type that's greater than or equal to the specified duration.

Syntax

template<class ToDuration, class Rep, class Period>
constexpr ToDuration
ceil(const duration<Rep, Period>& d);  // C++17

Parameters

ToDuration
The target duration type. Constrained as a specialization of duration.

Rep
The type of the internal representation of the source duration d.

Period
A std::ratio type representing the ratio of one second to the source Rep type (that is, seconds per Rep).

d
The source duration object.

Return value

Returns the smallest duration representable in ToDuration that's greater than or equal to the parameter d.

Remarks

ceil doesn't participate in overload resolution unless the ToDuration type is an instance of a duration.

`ceil(time_point)`

Returns the smallest time point representable in the target duration that's greater than or equal to the specified time point.

Syntax

template<class ToDuration, class Clock, class Duration>
constexpr time_point<Clock, ToDuration>
ceil(const time_point<Clock, Duration>& t);  // C++17

Parameters

ToDuration
The target duration type. Constrained as a specialization of duration.

Clock
The common clock type of the result and the source parameter tp.

Duration
The duration type of tp.

tp
The source time_point object.

Return value

Returns the smallest time point representable using ToDuration that's greater than or equal to tp. Effectively, time_point<Clock, ToDuration>(ceil<ToDuration>(tp.time_since_epoch()));.

Remarks

ceil doesn't participate in overload resolution unless the ToDuration type is an instance of a duration.

`clock_cast`

Converts a time_point for one clock to an equivalent time_point for another clock.

Syntax

template <class DestClock, class SourceClock, class Duration> 
auto clock_cast(const time_point<SourceClock, Duration>& t); // C++20

Parameters

DestClock
The clock type to convert the time_point to.

Duration
The duration of the SourceClock, or one that you specify.

SourceClock
The clock type that the time_point to convert is based on.

t
The time_point to convert.

Return value

A time_point equivalent to t, but specific to DestClock.

Remarks

The parameters SourceClock and Duration can be inferred via class template argument deduction when not explicitly passed. For example, given clock_cast<utc_clock>(file_clock::now()), SourceClock is deduced to be file_clock, and Duration is deduced to be file_clock::duration.

From the following list of well-formed clock conversions, the one that requires the fewest conversion steps to get from the SourceClock to the DestClock is selected.

clock_time_conversion<DestClock, SourceClock>{}(t)

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

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

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

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

For more information about what clock_time_conversion does, see clock_time_conversion struct.

Example `clock_cast`

// compile using: /std:c++latest
#include <iostream>
#include <chrono>

using namespace std::chrono;

int main()
{
    utc_clock::time_point t = clock_cast<utc_clock>(file_clock::now());
    std::cout << t;

    return 0;
}

2021-10-11 22:58:17.8540720

`current_zone`

Gets the current time zone object.

Syntax

const time_zone* current_zone();  // C++20

Return value

Returns a pointer to a time_zone as if by a call to get_tzdb().current_zone(). It throws a runtime_error exception if it's the first reference to the time zone database and the time zone database can't be initialized.

`duration_cast`

Casts a duration to the specified target duration type.

Syntax

template <class ToDuration, class Rep, class Period>
constexpr ToDuration
duration_cast(const duration<Rep, Period>& d);  // C++11

Parameters

ToDuration
The target duration type. Constrained as a specialization of duration.

Rep
The type of the internal representation of the source duration d.

Period
A std::ratio type representing the ratio of one second to the source Rep type (that is, seconds per Rep).

d
The source duration object to cast to the target duration type.

Return value

An object of type ToDuration that represents the duration d. It's truncated if necessary to fit into the target type. The result of casting a floating-point duration to an integral duration is undefined if the source contains a NaN, an infinity, or is too large for representation in the target duration.

Remarks

You don't need to use duration_cast to convert between duration types when the source period is exactly divisible by the target period, such as when you convert minutes to seconds. Also, you don't need it to convert between floating-point duration types. You can do both conversions by using ordinary casts or the duration constructors.

duration_cast doesn't participate in overload resolution unless ToDuration is an instance of duration. It does all conversions by using static_cast instead of implicit conversions. Multiplications and divisions are avoided if possible. For example, when the compiler knows that the common ratio of the target and source periods has a numerator or denominator of 1. Computations are done in the widest type available, then converted as if by static_cast to the result type when finished.

Example `duration_cast`

// compile using: /std:c++latest
#include <iostream>
#include <chrono>

using namespace std::chrono;

int main()
{
    seconds s(1);
    std::cout << duration_cast<microseconds>(s) << '\n';
    std::cout << duration_cast<nanoseconds>(s) << '\n';

    return 0;
}

1000000us
1000000000ns

`floor(duration)`

Returns the greatest representable duration in the target type that's less than or equal to the specified duration.

Syntax

template<class ToDuration, class Rep, class Period>
constexpr ToDuration
floor(const duration<Rep, Period>& d);  // C++17

Parameters

ToDuration
The target duration type. Constrained as a specialization of duration.

Rep
The type of the internal representation of the source duration d.

Period
A std::ratio type representing the ratio of one second to the source Rep type (that is, seconds per Rep).

d
The source duration object.

Return value

Returns the greatest duration representable in ToDuration that's less than or equal to the parameter d.

Remarks

floor doesn't participate in overload resolution unless the ToDuration type is an instance of a duration.

`floor(time_point)`

Returns the largest time point representable in the target duration that's less than or equal to the specified time point.

Syntax

template<class ToDuration, class Clock, class Duration>
constexpr time_point<Clock, ToDuration>
floor(const time_point<Clock, Duration>& tp);  // C++17

Parameters

ToDuration
The target duration type. Constrained as a specialization of duration.

Clock
The common clock type of the result and the source parameter tp.

Duration
The duration type of tp.

tp
The source time_point object.

Return value

Returns the largest time point representable using ToDuration that's less than or equal to tp. Effectively, time_point<Clock, ToDuration>(floor<ToDuration>(tp.time_since_epoch()));.

Remarks

floor doesn't participate in overload resolution unless the ToDuration type is an instance of a duration.

`from_stream`

Parse the input stream into one of the std::chrono time or interval types such as day, month, month_day, weekday, year, year_month, year_month_day, and so on, using the specified format.

If the parse fails, is.setstate(ios_base::failbit) is called and the output parameter isn't modified.

// 1) day - C++20
template<class charT class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(
    basic_istream<charT, traits>& is, const charT* fmt,
    day& d, basic_string<charT, traits, Alloc>* abbrev = nullptr,
    minutes* offset = nullptr);

// 2) duration - C++20
template<class charT, class traits, class Rep, class Period, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(
    basic_istream<charT, traits>& is, const charT* fmt,
    duration<Rep, Period>& dur, basic_string<charT, traits, Alloc>* abbrev = nullptr,
    minutes* offset = nullptr);

// 3) file_time - C++20
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,
    file_time<Duration>& ft, basic_string<charT, traits, Alloc>* abbrev = nullptr,
    minutes* offset = nullptr);

// 4) gps_time - C++20
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,
    gps_time<Duration>& gt, basic_string<charT, traits, Alloc>* abbrev = nullptr,
    minutes* offset = nullptr);

// 5) local_time - C++20
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,
    local_time<Duration>& lt, basic_string<charT, traits, Alloc>* abbrev = nullptr,
    minutes* offset = nullptr);

// 6) month - C++20
template<class charT, class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(
    basic_istream<charT, traits>& is, const charT* fmt,
    month& m, basic_string<charT, traits, Alloc>* abbrev = nullptr,
    minutes* offset = nullptr);

// 7) month_day - C++20
template<class charT, class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(
    basic_istream<charT, traits>& is, const charT* fmt,
    month_day& md, basic_string<charT, traits, Alloc>* abbrev = nullptr,
    minutes* offset = nullptr);

// 8) utc_time  - C++20
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>& ut, basic_string<charT, traits, Alloc>* abbrev = nullptr,
    minutes* offset = nullptr);

// 9) sys_time - C++20
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,
    sys_time<Duration>& st, basic_string<charT, traits, Alloc>* abbrev = nullptr,
    minutes* offset = nullptr);

// 10) tai_time - C++20
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,
    tai_time<Duration>& tt, basic_string<charT, traits, Alloc>* abbrev = nullptr,
    minutes* offset = nullptr);

// 11) weekday - C++20
template<class charT, class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(
    basic_istream<charT, traits>& is, const charT* fmt,
    weekday& wd, basic_string<charT, traits, Alloc>* abbrev = nullptr,
    minutes* offset = nullptr);

// 12) year - C++20
template<class charT, class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(
    basic_istream<charT, traits>& is, const charT* fmt,
    year& y, basic_string<charT, traits, Alloc>* abbrev = nullptr,
    minutes* offset = nullptr);

// 13) year_month - C++20
template<class charT, class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(
    basic_istream<charT, traits>& is, const charT* fmt,
    year_month& ym, basic_string<charT, traits, Alloc>* abbrev = nullptr,
    minutes* offset = nullptr);

// 14) year_month_day - C++20
template<class charT, class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(
    basic_istream<charT, traits>& is, const charT* fmt,
    year_month_day& ymd, basic_string<charT, traits, Alloc>* abbrev = nullptr,
    minutes* offset = nullptr);

Template parameters

Alloc
The type that represents the allocator object that handles the string's allocation and deallocation of memory.

charT
The data type of a single character to be read from the stream and stored in the string. The C++ Standard Library provides specializations of this class template, with the type definitions string for elements of type char, wstring, for wchar_t, u16string for char16_t, and u32string for char32_t.

traits
Describes charT attributes for the basic_string and basic_istream specialization.

Rep
The internal representation type of a duration type.

Period
A std::ratio type representing the ratio of one second to the source Rep type (that is, seconds per Rep).

Duration
The duration type used for time specialization.

Parameters

abbrev
If abbrev isn't nullptr, and the format specifier %Z is specified, and the parse is successful, then abbrev contains the parsed value.

d
If the parse is successful, contains the parsed day when the function returns.

dur
The duration parsed from the stream.

fmt
The format string used to match the input. See Parse format strings for the list of parse formatting options.

ft
The file_time parsed from the stream.

gt
The gps_time parsed from the stream.

is
The input stream to parse.

lt
The local_time parsed from the stream.

m
The month parsed from the stream.

md
The month_day parsed from the stream.

offset
If offset isn't nullptr, and the format specifier %z or modified variant such as %Ez or %0z is specified, and the parse is successful, then offset points to the parsed value.

st
The sys_time parsed from the stream.

tt
The tai_time parsed from the stream.

ut
The utc_time parsed from the stream.

wd
The weekday parsed from the stream.

y
The year parsed from the stream.

ym
The year_month parsed from the stream.

ymd
The year_month_day parsed from the stream.

Return value

The input stream, is

Example: `from_stream`

// compile using: /std:c++latest
#include <chrono>
#include <iostream>

int main()
{
    std::istringstream str{ "22" };
    std::basic_istream<char> stream{ str.rdbuf() };
    std::chrono::day d;
    std::chrono::from_stream(stream, "%d", d);
    std::cout << d << "\n";
    return 0;
}

Remarks

7) If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null. If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.

12) If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null. If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.

`from_stream` format strings

The format may be one of these strings:

Date

Specifier	Description
`%D`	Equivalent to `%m/%d/%y`
`%F` `%`N`F`	Equivalent to `%Y-%m-%d`. If modified with a width `N`, the width is applied to only `%Y`.
`%x` `%Ex`	The locale's date representation. `%Ex` parses the locale's alternate date representation.¹

Day

Specifier	Description
`%d` `%Od` `%`N`d` `%e` `%Oe` `%`N`e`	The day of the month as a decimal number. `%`N`d` specifies the maximum number of characters to read, for example `%1d`. If N isn't specified, the default is 2. Leading zeroes are permitted but not required. `%Od` (letter `O`, not zero) interprets the locale's alternative representation of the day of the month.¹ `%e` is equivalent to `%d` and can be modified like `%d`.¹

Day of the week

Specifier	Description
`%a` `%A`	The locale's full or abbreviated case-insensitive weekday name. `%A` is equivalent to `%a`
`%u` `%`N`u`	The ISO weekday as a decimal number (1-7), where Monday is 1. `%`N`u` specifies the maximum number of characters to read, for example `%2u`. If N isn't specified, the default is 1. Leading zeros are permitted but not required.
`%w` `%`N`w` `%Ow`	The weekday as a decimal number (0-6), where Sunday is 0. `%`N`w` specifies the maximum number of characters to read, for example `%2w`. If N isn't specified, the default is 1. Leading zeroes are permitted but not required. `%Ow` (letter `O`, not zero) interprets the locale's alternative representation.¹

Week/day of the year

Specifier	Description
`%j` `%`N`j`	If the type being formatted is a specialization of duration, the decimal number of days without padding. Otherwise, the day of the year as a decimal number. `Jan 1` is `001`. If the result is fewer than three digits, it's left-padded with `0` (zero) to three digits. `%`N`j`specifies the maximum number of characters to read, for example `%2j`. If N isn't specified, the default is 3. Leading digits are allowed but aren't required.
`%U` `%`N`U` `%OU`	The week number of the year as a decimal number. The first Sunday of the year is the first day of week `01`. Days of the same year before that week are in week `00`. If the result is a single digit, it's prefixed with `0` (zero). `%`N`U` specifies the maximum number of characters to read, for example `%2U`. If N isn't specified, the default is 2. Leading zeros are permitted but no required. `%OU` (letter `O`, not zero) parses the locale's alternative representation.¹
`%W` `%`N`W` `%OW`	The week number of the year as a decimal number. The first Monday of the year is the first day of week `01`. Days of the same year before that week are in week `00`. If the result is a single digit, it's prefixed with `0` (zero). `%`N`W` specifies the maximum number of characters to read, for example `%2W`. If N isn't specified, the default is 1 Leading zeros are permitted but no required.`%OW` (letter `O`, not zero) parses the locale's alternative representation.¹

Time of day

Specifier	Description
`%H` `%`N`H` `%OH`	The hour (24-hour clock) as a decimal number. If the result is a single digit, it's prefixed with a `0` (zero). `%`N`H` specifies the maximum number of characters to read, for example, `%1H`. If N isn't specified, the default is 2. Leading zeroes are permitted but not required. `%OH` (letter `O`, not zero) parses the locale's alternative representation.¹
`%I` `%`N`I` `%OI`	The hour (12-hour clock) as a decimal number. If the result is a single digit, it's prefixed with `0` (zero). `%`N`I` specifies the maximum number of characters to read, for example, `%1I`. If N isn't specified, the default is 2. Leading zeroes are permitted but not required. `%OI` (letter `O`, not zero) parses the locale's alternative representation.¹
`%M` `%`N`M` `%OM`	The minutes as a decimal number. If the result is a single digit, it's prefixed with `0` (zero). `%`N`M` specifies the maximum number of characters to read, for example `%3M`. If N isn't specified, the default is 2. Leading zeroes are permitted but not required. `%OM` (letter `O`, not zero) parses the locale's alternative representation.¹
`%S` `%`N`S` `%OS`	Seconds as a decimal number. If the number of seconds is less than 10, the result is prefixed with `0` (zero). If the precision of the input can't be exactly represented with seconds, then the format is a decimal floating-point number with a fixed format. It has a microseconds precision if the function can't convert the floating-point decimal seconds within 18 fractional digits. Otherwise, its precision matches the precision of the input. The character for the decimal point is localized according to the locale. `%`N`S` specifies the maximum number of characters to read, for example `%3S`. If N isn't specified, the default is 2. Leading zeroes are permitted but not required. `%OS` (letter `O`, not zero) parses the locale's alternative representation.¹
`%p`	The locale's equivalent of the AM/PM designations associated with a 12-hour clock.
`%r`	The locale's 12-hour clock time.
`%R`	Equivalent to `%H:%M`.
`%T`	Equivalent to `"%H:%M:%S"`.
`%X`, `%EX`	The locale's time representation. `%EX` parses the alternate locale's time representation.¹

Month

Specifier	Description
`%b`, `%B`, `%h`	The locale's full or abbreviated month name. If the value doesn't contain a valid month, a `format_error` exception is thrown. `%h` is equivalent to `%b`.
`%m`, `%`N`m`, `%Om`	The month as a decimal number. Jan is 1. `%`N`m` specifies the maximum number of characters to read, for example, `%3m`. If N isn't specified, the default is 2. Leading zeroes are permitted but not required. `%Om` (letter `O`, not zero) interprets the locale's alternative representation.¹

Year

Specifier	Description
`%C`, `%`N`C`, `%EC`	The century as a decimal number. `%`N`C` specifies the maximum number of characters to read, for example, `%1N`. If N isn't specified, the default is 2. Leading zeroes are permitted but not required. `%EC` interprets the locale's alternative representation of the century.
`%y`, `%`N`y`, `%Ey`, `%Oy`	The last two decimal digits of the year. If the century isn't otherwise specified (for example, by using `%C`), values in the range `[69, 99]` are presumed to refer to the years 1969 to 1999, and values in the range `[00, 68]` are presumed to refer to the years 2000 to 2068. `%`N`y` specifies the maximum number of characters to read. If N isn't specified, the default is 2. Leading zeroes are permitted but not required. `%Ey` and `%Oy` (letter `O`, not zero) interpret the locale's alternative representation.¹
`%Y`, `%`N`Y`, `%EY`,	The year as a decimal number. If the result is fewer than four digits, it's left-padded with `0` (zero) to four digits. `%`N`Y` specifies the maximum number of characters to read. If N isn't specified, the default is 4. `%EY` parses the locale's alternative full year representation.¹

ISO 8601 week-based year

In ISO 8601, weeks begin with Monday. The first week of the year must include January 4 and include the first Thursday of the year.

Specifier	Replacement
`%g` `%`N`g`	The last two decimal digits of the ISO week-based year. If the result is a single digit, is prefixed by `0` (zero). `%`N`g` specifies the maximum number of characters to read, for example, `%1g`. If N isn't specified, the default is 2
`%G` `%`N`G`	The ISO week-based year as a decimal number. If the result is fewer than four digits, it's left-padded with `0` (zero) to four digits. `%`N`G` specifies the maximum number of characters to read, for example, `%1G`. If N isn't specified, the default is 4
`%V` `%OV` `%`N`V`	The ISO week-based week number as a decimal number. If the result is a single digit, it's prefixed with `0` (zero). `%`N`V` specifies the maximum number of characters to read, for example, `%1V`. If N isn't specified, the default is 2 `%OV` (letter `O`, not zero) parses the locale's alternative representation.¹

General

Specifier	Replacement
`%%`	Matches the % character
`%c` `%Ec`	The locale's date and time representation. `%Ec` interprets the locale's alternate date and time representation.¹
`%n`	Matches a new-line character
`%t`	Matches zero or one whitespace character
`%z` `%Ez` `%Oz`	The offset from UTC in the format `[+\|-]hh[mm]`. For example, `-0430` refers to 4 hours 30 minutes behind UTC, and `04` refers to 4 hours ahead of UTC. `%Ez` and `%Oz` (letter `O`, not zero) parse a `:` between the hours and minutes and render leading zeroes on the hour field optional¹: `[+\|-]h[h][:mm]`. For example, `-04:30` refers to 4 hours 30 minutes behind UTC, and 4 refers to 4 hours ahead of UTC.
`%Z`	The time zone abbreviation or name. A single word is parsed. This word can only contain alphanumeric characters from the basic source character set, or one of `_`, `/`, `-`, or `+`.

Flags by type

Class	Specifier/Flag
`day`	d, e
`duration`	j, H, I, M, S, r, R, T, p, (q, Q are only for format, not parsing)
`file_time`	Z, z, c, x, X, D, F, g, G, j, U, V, W, Y, y, C, b, h, B, m, d, e, a, A, u, w, H, I, M, S, r, R, T, p
`gps_time`	Z, z, c, x, X, D, F, g, G, j, U, V, W, Y, y, C, b, h, B, m, d, e, a, A, u, w, H, I, M, S, r, R, T, p
`hh_mm_ss`	H, I, M, S, r, R, T, p
`local_time`	c, x, X, D, F, g, G, j, U, V, W, Y, y, C, b, h, B, m, d, e, a, A, u, w, H, I, M, S, r, R, T, p
`local_time_format_t`	z, Z, c, x, X, D, F, Y, C, y, b, B, h, m, d, e, a, A, u, w, H, I, M, S, r, R, T, p, g, G, U, V, W
`local_info`	z, Z
`month`	b, h, B, m
`month_day`	B, d, j, e, b, h, m
`month_day_last`	B, d, j, e, b, h, m
`month_weekday`	b, B, h, m, a, A, u, w
`month_weekday_last`	b, B, h, m, a, A, u, w
`sys_info`	z, Z
`sys_time`	Z, z, c, x, X, D, F, g, G, j, U, V, W, Y, y, C, b, h, B, m, d, e, a, A, u, w, H, I, M, S, r, R, T, p
`tai`	Z, z, c, x, X, D, F, g, G, j, U, V, W, Y, y, C, b, h, B, m, d, e, a, A, u, w, H, I, M, S, r, R, T, p
`utc_time`	Z, z, c, x, X, D, F, g, G, j, U, V, W, Y, y, C, b, h, B, m, d, e, a, A, u, w, H, I, M, S, r, R, T, p
`weekday`	a, A, u, w
`weekday_indexed`	a, A, u, w
`weekday_last`	a, A, u, w
`year`	Y, y, C
`year_month`	Y, y, B, g, G, h, C, b, m
`year_month_day`	D, F, g, G, j, U, V, W, Y, y, C, b, h, B, m, d, e, a, A, u, w
`year_month_day_last`	D, F, g, G, j, U, V, W, Y, y, C, b, h, B, m, d, e, a, A, u, w
`year_month_weekday`	D, F, g, G, j, U, V, W, Y, y, C, b, h, B, m, d, e, a, A, u, w
`year_month_weekday_last`	D, F, g, G, j, U, V, W, Y, y, C, b, h, B, m, d, e, a, A, u, w
`zoned_time`	z, Z, c, x, X, D, F, Y, C, y, b, B, h, m, d, e, a, A, u, w, H, I, M, S, r, R, T, p, g, G, U, V, W

¹The implementation is currently consistent with strftime in that although O (letter O) and e are accepted, they're ignored. That is, "%Od" is interpreted as "%d".

`get_leap_second_info`

Returns a leap_second_info for the specified time. This provides info about whether the supplied time occurs during a leap second insertion. It also provides the number of leap seconds that have been added between January 1, 1970 and the specified time. A leap second insertion occurs when the International Earth Rotation and Reference Systems Service (IERS) declares that a leap second will be added (resulting in a 61 second minute) to account for the difference between atomic time and time tracked by measuring the rotation of the earth, which is irregular, and is gradually slowing.

Syntax

template<class Duration>
leap_second_info
get_leap_second_info(const utc_time<Duration>& ut);  // C++20

Parameters

ut
The source utc_time to get the leap_second_info for.

Return value

Returns a leap_second_info whose member is_leap_second is true if ut is during a positive leap second insertion; otherwise, false. The elapsed member holds the sum of leap seconds between the epoch date 1970-01-01and ut. If is_leap_second is true, the leap second referred to by ut is included in the elapsed sum.

`get_tzdb`

Gets the first entry in the program-wide time zone database list.

Syntax

const tzdb& get_tzdb();  // C++20

Return value

Returns a reference to the first tzdb object as if by a call to get_tzdb_list().front(). It throws a runtime_error exception if it's the first reference to the time zone database and the time zone database can't be initialized.

Remarks

If it hasn't been initialized yet, get_tzdb initializes the program-wide time zone database on first access. Upon initialization, the database is a tzdb_list that holds a single initialized tzdb object.

`get_tzdb_list`

Gets the program-wide singleton time zone database.

Syntax

tzdb_list& get_tzdb_list();  // C++20

Return value

Returns a reference to the program-wide tzdb_list object. It throws a runtime_error exception if it can't return a reference to a valid tzdb_list object.

Remarks

If it hasn't been initialized yet, get_tzdb_list initializes the program-wide time zone database on first access. Upon initialization, the database is a tzdb_list that holds a single initialized tzdb object. The get_tzdb_list function is thread-safe.

`is_am`

Predicate for whether the specified hour is in the ante-meridiem (AM) part of the day.

Syntax

constexpr bool is_am(const hours& h) noexcept;  // C++20

Parameters

h
The source hours representation in 24-hour time notation.

Return value

Returns true if 0h <= h && h <= 11h, otherwise false.

`is_pm`

Predicate for whether the specified hour is in the post-meridiem (PM) part of the day.

Syntax

constexpr bool is_pm(const hours& h) noexcept;  // C++20

Parameters

h
The source hours representation in 24-hour time notation.

Return value

Returns true if 12h <= h && h <= 23h, otherwise false.

`locate_zone`

Gets a time zone object specified by time zone name.

Syntax

const time_zone* locate_zone(string_view tz_name);  // C++20

Parameters

tz_name
The name of the time zone to return.

Return value

Returns a pointer to a time_zone as if by a call to get_tzdb().locate_zone(tz_name). It throws a runtime_error exception if it can't find the specified time zone, or if it's the first reference to the time zone database and the time zone database can't be initialized.

Remarks

If it hasn't been initialized yet, locate_zone initializes the program-wide time zone database on first access. Upon initialization, the database is a tzdb_list that holds a single initialized tzdb object.

`make12`

Returns the specified hour in 12-hour time notation.

Syntax

constexpr hours make12(const hours& h) noexcept;  // C++20

Parameters

h
The source hours representation in 24-hour time notation.

Return value

Returns the 12-hour equivalent of h in the range [1h, 12h]. The return value is unspecified if h isn't in the range [0h, 23h].

`make24`

Returns the specified hour in 24-hour time notation.

Syntax

constexpr hours make24(const hours& h, bool is_pm) noexcept;  // C++20

Parameters

h
The source hours representation in 12-hour time notation.

is_pm
The source hours representation is PM (if true) or AM (if false).

Return value

If is_pm is false, make24 returns the 24-hour equivalent of h in the range [0h, 11h], assuming h represents an AM hour. Otherwise, it returns the 24-hour equivalent of h in the range [12h, 23h], assuming h represents a PM hour. The return value is unspecified if h isn't in the range [1h, 12h].

`reload_tzdb`

Reloads the time zone database if updated information is available.

Syntax

const tzdb& reload_tzdb();

Return value

After it makes a time zone database update, if any, reload_tzdb returns a reference to the first tzdb object as if by a call to get_tzdb_list().front(). It throws a runtime_error exception if it can't return a reference to a valid tzdb object.

Remarks

The local time zone database is the one supplied when the program first accesses the database, such as by a call to current_zone. While the program is running, the implementation may update the time zone database. The update doesn't affect the program in any way unless the program calls the reload_tzdb function. The potentially updated time zone database is called the remote time zone database.

The reload_tzdb function checks the version of both the local time zone database and the remote time zone database. If the versions of the local and remote databases are the same, it makes no changes. Otherwise, the remote database is pushed to the front of the tzdb_list accessed by get_tzdb_list. An update doesn't invalidate any pointers, references, or iterators. The reload_tzdb function is thread-safe for calls to get_tzdb_list().front() and get_tzdb_list().erase_after().

`remote_version`

Gets a string that contains the latest remote database version.

Syntax

string remote_version();

Return value

Returns a string that contains the latest remote database version.

`round(duration)`

Rounds the specified duration to the nearest representable duration in the target type.

Syntax

template<class ToDuration, class Rep, class Period>
constexpr ToDuration
round(const duration<Rep, Period>& d);  // C++17

Parameters

ToDuration
The target duration type. Constrained as a specialization of duration where treat_as_floating_point_v<typename ToDuration::rep> is false.

Rep
The type of the internal representation of the source duration d.

Period
A std::ratio type representing the ratio of one second to the source Rep type (that is, seconds per Rep).

d
The source duration object.

Return value

Returns the nearest duration representable in ToDuration to the parameter d. Ties go to the even value, that is, the value t where t % 2 == 0.

Remarks

round doesn't participate in overload resolution unless the ToDuration type is an instance of a duration, and ToDuration has an integral representation.

`round(time_point)`

Returns the nearest time point representable in the target duration to the specified time point.

template<class ToDuration, class Clock, class Duration>
constexpr time_point<Clock, ToDuration>
round(const time_point<Clock, Duration>& tp);

Parameters

ToDuration
The target duration type. Constrained as a specialization of duration where treat_as_floating_point_v<typename ToDuration::rep> is false.

Clock
The common clock type of the result and the source parameter tp.

Duration
The duration type of tp.

tp
The source time_point object.

Return value

Returns the nearest time point representable using ToDuration to tp. Ties go to the even value, that is, the value t where t % 2 == 0. Effectively, time_point<Clock, ToDuration>(round<ToDuration>(tp.time_since_epoch()));.

Remarks

round doesn't participate in overload resolution unless the ToDuration type is an instance of a duration.

`time_point_cast`

Casts a time_point object to a time_point that has a different duration type.

Syntax

template <class ToDuration, class Clock, class Duration>
time_point<Clock, ToDuration>
time_point_cast(const time_point<Clock, Duration>& tp);  // C++11

Parameters

ToDuration
The target duration type. Constrained as a specialization of duration.

Clock
The common clock type of the result and the source parameter tp.

Duration
The duration type of tp.

tp
The time_point object to cast to one that has the ToDuration type.

Return value

Returns a time_point object that has a ToDuration type. Effectively, time_point<Clock, ToDuration>(duration_cast<ToDuration>(t.time_since_epoch()));.

Remarks

Unless ToDuration is a specialization of duration, this function doesn't participate in overload resolution.

<chrono> functions

abs(duration)

Syntax

Parameters

Return value

Example: abs(duration)

ceil(duration)

Syntax

Parameters

Return value

Remarks

ceil(time_point)

Syntax

Parameters

Return value

Remarks

clock_cast

Syntax

Parameters

Return value

Remarks

Example clock_cast

current_zone

Syntax

Return value

duration_cast

Syntax

Parameters

Return value

Remarks

Example duration_cast

floor(duration)

Syntax

Parameters

Return value

Remarks

floor(time_point)

Syntax

Parameters

Return value

Remarks

from_stream

Template parameters

Parameters

Return value

Example: from_stream

Remarks

from_stream format strings

Date

Day

Day of the week

Week/day of the year

Time of day

Month

Year

ISO 8601 week-based year

General

Flags by type

get_leap_second_info

Syntax

Parameters

Return value

get_tzdb

Syntax

Return value

Remarks

get_tzdb_list

Syntax

Return value

Remarks

is_am

Syntax

Parameters

Return value

is_pm

Syntax

Parameters

Return value

locate_zone

Syntax

`<chrono>` functions

`abs(duration)`

Example: `abs(duration)`

`ceil(duration)`

`ceil(time_point)`

`clock_cast`

Example `clock_cast`

`current_zone`

`duration_cast`

Example `duration_cast`

`floor(duration)`

`floor(time_point)`

`from_stream`

Example: `from_stream`

`from_stream` format strings

`get_leap_second_info`

`get_tzdb`

`get_tzdb_list`

`is_am`

`is_pm`

`locate_zone`

`make12`

`make24`

`reload_tzdb`

`remote_version`

`round(duration)`

`round(time_point)`

`time_point_cast`