[time.cal.day]

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

## 30.8 The civil calendar [[time.cal]](time.cal#day)

### 30.8.3 Class day [time.cal.day]

#### [30.8.3.1](#overview) Overview [[time.cal.day.overview]](time.cal.day.overview)

namespace std::chrono {class day {unsigned char d_; // *exposition only*public: day() = default; constexpr explicit day(unsigned d) noexcept; constexpr day& operator++() noexcept; constexpr day operator++(int) noexcept; constexpr day& operator--() noexcept; constexpr day operator--(int) noexcept; constexpr day& operator+=(const days& d) noexcept; constexpr day& operator-=(const days& d) noexcept; constexpr explicit operator unsigned() const noexcept; constexpr bool ok() const noexcept; };}

[1](#overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3968)

day represents a day of a month[.](#overview-1.sentence-1)

It normally holds values in the range 1 to 31,
but may hold non-negative values outside this range[.](#overview-1.sentence-2)

It can be constructed with any unsigned value,
which will be subsequently truncated to fit into day's unspecified internal storage[.](#overview-1.sentence-3)

day meets the [*Cpp17EqualityComparable*](utility.arg.requirements#:Cpp17EqualityComparable "16.4.4.2 Template argument requirements [utility.arg.requirements]") (Table [28](utility.arg.requirements#tab:cpp17.equalitycomparable "Table 28: Cpp17EqualityComparable requirements"))
and [*Cpp17LessThanComparable*](utility.arg.requirements#:Cpp17LessThanComparable "16.4.4.2 Template argument requirements [utility.arg.requirements]") (Table [29](utility.arg.requirements#tab:cpp17.lessthancomparable "Table 29: Cpp17LessThanComparable requirements")) requirements,
and participates in basic arithmetic with days objects,
which represent a difference between two day objects[.](#overview-1.sentence-4)

[2](#overview-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3979)

day is a trivially copyable and standard-layout class type[.](#overview-2.sentence-1)

#### [30.8.3.2](#members) Member functions [[time.cal.day.members]](time.cal.day.members)

[🔗](#lib:day,constructor)

`constexpr explicit day(unsigned d) noexcept;
`

[1](#members-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3990)

*Effects*: Initializes d_ with d[.](#members-1.sentence-1)

The value held is unspecified if d is not in the range [0, 255][.](#members-1.sentence-2)

[🔗](#lib:operator++,day)

`constexpr day& operator++() noexcept;
`

[2](#members-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4002)

*Effects*: ++d_[.](#members-2.sentence-1)

[3](#members-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4006)

*Returns*: *this[.](#members-3.sentence-1)

[🔗](#lib:operator++,day_)

`constexpr day operator++(int) noexcept;
`

[4](#members-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4017)

*Effects*: ++(*this)[.](#members-4.sentence-1)

[5](#members-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4021)

*Returns*: A copy of *this as it existed on entry to this member function[.](#members-5.sentence-1)

[🔗](#lib:operator--,day)

`constexpr day& operator--() noexcept;
`

[6](#members-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4032)

*Effects*: Equivalent to: --d_[.](#members-6.sentence-1)

[7](#members-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4036)

*Returns*: *this[.](#members-7.sentence-1)

[🔗](#lib:operator--,day_)

`constexpr day operator--(int) noexcept;
`

[8](#members-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4047)

*Effects*: --(*this)[.](#members-8.sentence-1)

[9](#members-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4051)

*Returns*: A copy of *this as it existed on entry to this member function[.](#members-9.sentence-1)

[🔗](#lib:operator+=,day)

`constexpr day& operator+=(const days& d) noexcept;
`

[10](#members-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4062)

*Effects*: *this = *this + d[.](#members-10.sentence-1)

[11](#members-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4066)

*Returns*: *this[.](#members-11.sentence-1)

[🔗](#lib:operator-=,day)

`constexpr day& operator-=(const days& d) noexcept;
`

[12](#members-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4077)

*Effects*: *this = *this - d[.](#members-12.sentence-1)

[13](#members-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4081)

*Returns*: *this[.](#members-13.sentence-1)

[🔗](#lib:operator_unsigned,day)

`constexpr explicit operator unsigned() const noexcept;
`

[14](#members-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4092)

*Returns*: d_[.](#members-14.sentence-1)

[🔗](#lib:ok,day)

`constexpr bool ok() const noexcept;
`

[15](#members-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4103)

*Returns*: 1 <= d_ && d_ <= 31[.](#members-15.sentence-1)

#### [30.8.3.3](#nonmembers) Non-member functions [[time.cal.day.nonmembers]](time.cal.day.nonmembers)

[🔗](#lib:operator==,day)

`constexpr bool operator==(const day& x, const day& y) noexcept;
`

[1](#nonmembers-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4116)

*Returns*: unsigned{x} == unsigned{y}[.](#nonmembers-1.sentence-1)

[🔗](#lib:operator%3c=%3e,day)

`constexpr strong_ordering operator<=>(const day& x, const day& y) noexcept;
`

[2](#nonmembers-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4127)

*Returns*: unsigned{x} <=> unsigned{y}[.](#nonmembers-2.sentence-1)

[🔗](#lib:operator+,day)

`constexpr day operator+(const day& x, const days& y) noexcept;
`

[3](#nonmembers-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4138)

*Returns*: day(unsigned{x} + y.count())[.](#nonmembers-3.sentence-1)

[🔗](#lib:operator+,day_)

`constexpr day operator+(const days& x, const day& y) noexcept;
`

[4](#nonmembers-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4149)

*Returns*: y + x[.](#nonmembers-4.sentence-1)

[🔗](#lib:operator-,day)

`constexpr day operator-(const day& x, const days& y) noexcept;
`

[5](#nonmembers-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4160)

*Returns*: x + -y[.](#nonmembers-5.sentence-1)

[🔗](#lib:operator-,day_)

`constexpr days operator-(const day& x, const day& y) noexcept;
`

[6](#nonmembers-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4171)

*Returns*: days{int(unsigned{x}) - int(unsigned{y})}[.](#nonmembers-6.sentence-1)

[🔗](#lib:operator%3c%3c,day)

`template<class charT, class traits>
  basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const day& d);
`

[7](#nonmembers-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4184)

*Effects*: Equivalent to:return os << (d.ok() ? format(*STATICALLY-WIDEN*<charT>("{:%d}"), d) : format(*STATICALLY-WIDEN*<charT>("{:%d} is not a valid day"), d));

[🔗](#lib:from_stream,day)

`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);
`

[8](#nonmembers-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4204)

*Effects*: Attempts to parse the input stream is into the day d using
the format flags given in the NTCTS fmt as specified in [[time.parse]](time.parse "30.13 Parsing")[.](#nonmembers-8.sentence-1)

If the parse fails to decode a valid day,is.setstate(ios_base​::​failbit) is called andd is not modified[.](#nonmembers-8.sentence-2)

If %Z is used and successfully parsed,
that value will be assigned to *abbrev if abbrev is non-null[.](#nonmembers-8.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-8.sentence-4)

[9](#nonmembers-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4218)

*Returns*: is[.](#nonmembers-9.sentence-1)

[🔗](#lib:operator%22%22d,day)

`constexpr chrono::day operator""d(unsigned long long d) noexcept;
`

[10](#nonmembers-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L4229)

*Returns*: day{static_cast<unsigned>(d)}[.](#nonmembers-10.sentence-1)