27 Time library [time]

27.8 The civil calendar [time.cal]

27.8.4 Class month [time.cal.month]

27.8.4.3 Non-member functions [time.cal.month.nonmembers]

constexpr bool operator==(const month& x, const month& y) noexcept;
Returns: unsigned{x} == unsigned{y}.
constexpr strong_ordering operator<=>(const month& x, const month& y) noexcept;
Returns: unsigned{x} <=> unsigned{y}.
constexpr month operator+(const month& x, const months& y) noexcept;
Returns:
month{modulo(static_cast<long long>(unsigned{x}) + (y.count() - 1), 12) + 1}
where modulo(n, 12) computes the remainder of n divided by 12 using Euclidean division.
Note
:
Given a divisor of 12, Euclidean division truncates towards negative infinity and always produces a remainder in the range of [0, 11].
Assuming no overflow in the signed summation, this operation results in a month holding a value in the range [1, 12] even if !x.ok().
— end note
 ]
Example
:
February + months{11} == January.
— end example
 ]
constexpr month operator+(const months& x, const month& y) noexcept;
Returns: y + x.
constexpr month operator-(const month& x, const months& y) noexcept;
Returns: x + -y.
constexpr months operator-(const month& x, const month& y) noexcept;
Returns: If x.ok() == true and y.ok() == true, returns a value m in the range [months{0}, months{11}] satisfying y + m == x.
Otherwise the value returned is unspecified.
Example
:
January - February == months{11}.
— end example
 ]
template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const month& m);
Effects: Equivalent to:
return os << (m.ok() ?
  format(os.getloc(), STATICALLY-WIDEN<charT>("{:%b}"), m) :
  format(os.getloc(), STATICALLY-WIDEN<charT>("{} is not a valid month"),
         static_cast<unsigned>(m)));
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);
Effects: Attempts to parse the input stream is into the month m using the format flags given in the NTCTS fmt as specified in [time.parse].
If the parse fails to decode a valid month, is.setstate(ios_­base​::​failbit) shall be called and m 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.