27 Time library [time]

27.8 The civil calendar [time.cal]

27.8.6 Class weekday [time.cal.wd]

27.8.6.3 Non-member functions [time.cal.wd.nonmembers]

constexpr bool operator==(const weekday& x, const weekday& y) noexcept;
Returns: x.wd_­ == y.wd_­.
constexpr weekday operator+(const weekday& x, const days& y) noexcept;
Returns:
weekday{modulo(static_cast<long long>(x.wd_) + y.count(), 7)}
where modulo(n, 7) computes the remainder of n divided by 7 using Euclidean division.
Note
:
Given a divisor of 7, Euclidean division truncates towards negative infinity and always produces a remainder in the range of [0, 6].
Assuming no overflow in the signed summation, this operation results in a weekday holding a value in the range [0, 6] even if !x.ok().
— end note
 ]
Example
:
Monday + days{6} == Sunday.
— end example
 ]
constexpr weekday operator+(const days& x, const weekday& y) noexcept;
Returns: y + x.
constexpr weekday operator-(const weekday& x, const days& y) noexcept;
Returns: x + -y.
constexpr days operator-(const weekday& x, const weekday& y) noexcept;
Returns: If x.ok() == true and y.ok() == true, returns a value d in the range [days{0}, days{6}] satisfying y + d == x.
Otherwise the value returned is unspecified.
Example
:
Sunday - Monday == days{6}.
— end example
 ]
template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const weekday& wd);
Effects: Equivalent to:
return os << (wd.ok() ?
  format(os.getloc(), STATICALLY-WIDEN<charT>("{:%a}"), wd) :
  format(os.getloc(), STATICALLY-WIDEN<charT>("{} is not a valid weekday"),
         static_cast<unsigned>(wd.wd_)));
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);
Effects: Attempts to parse the input stream is into the weekday wd using the format flags given in the NTCTS fmt as specified in [time.parse].
If the parse fails to decode a valid weekday, is.setstate(ios_­base​::​failbit) shall be called and wd shall not be modified.
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.
Returns: is.