27 Time library [time]

27.8 The civil calendar [time.cal]

27.8.14 Class year_­month_­day [time.cal.ymd]

27.8.14.3 Non-member functions [time.cal.ymd.nonmembers]

constexpr bool operator==(const year_month_day& x, const year_month_day& y) noexcept;
Returns: x.year() == y.year() && x.month() == y.month() && x.day() == y.day().
constexpr strong_ordering operator<=>(const year_month_day& x, const year_month_day& y) noexcept;
Effects: Equivalent to:
if (auto c = x.year() <=> y.year(); c != 0) return c;
if (auto c = x.month() <=> y.month(); c != 0) return c;
return x.day() <=> y.day();
constexpr year_month_day operator+(const year_month_day& ymd, const months& dm) noexcept;
Returns: (ymd.year() / ymd.month() + dm) / ymd.day().
Note
:
If ymd.day() is in the range [1d, 28d], ok() will return true for the resultant year_­month_­day.
— end note
 ]
constexpr year_month_day operator+(const months& dm, const year_month_day& ymd) noexcept;
Returns: ymd + dm.
constexpr year_month_day operator-(const year_month_day& ymd, const months& dm) noexcept;
Returns: ymd + (-dm).
constexpr year_month_day operator+(const year_month_day& ymd, const years& dy) noexcept;
Returns: (ymd.year() + dy) / ymd.month() / ymd.day().
Note
:
If ymd.month() is February and ymd.day() is not in the range [1d, 28d], ok() may return false for the resultant year_­month_­day.
— end note
 ]
constexpr year_month_day operator+(const years& dy, const year_month_day& ymd) noexcept;
Returns: ymd + dy.
constexpr year_month_day operator-(const year_month_day& ymd, const years& dy) noexcept;
Returns: ymd + (-dy).
template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const year_month_day& ymd);
Effects: Equivalent to:
return os << (ymd.ok() ?
  format(STATICALLY-WIDEN<charT>("{:%F}"), ymd) :
  format(STATICALLY-WIDEN<charT>("{:%F} is not a valid date"), ymd));
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);
Effects: Attempts to parse the input stream is into the year_­month_­day ymd using the format flags given in the NTCTS fmt as specified in [time.parse].
If the parse fails to decode a valid year_­month_­day, is.setstate(ios_­base​::​failbit) shall be called and ymd 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.