constexpr bool operator==(const weekday& x, const weekday& y) noexcept;
constexpr weekday operator+(const weekday& x, const days& y) noexcept;
constexpr weekday operator+(const days& x, const weekday& y) noexcept;
constexpr weekday operator-(const weekday& x, const days& y) noexcept;
constexpr days operator-(const weekday& x, const weekday& y) noexcept;
template<class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const weekday& 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);