# 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: unsigned{x} == unsigned{y}.
```constexpr weekday operator+(const weekday& x, const days& y) noexcept; ```
Returns:
```weekday{modulo(static_cast<long long>(unsigned{x}) + y.count(), 7)}
```
where modulo(n, 7) computes the remainder of n divided by 7 using Euclidean division.
[â€‰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().
â€”â€‰end note
â€‰]
[â€‰Example
:
Monday + days{6} == Sunday.
â€”â€‰end example
â€‰]
```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.
[â€‰Example
:
Sunday - Monday == days{6}.
â€”â€‰end example
â€‰]
```template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const weekday& wd); ```
Effects: If wd.ok() == true inserts format(os.getloc(), fmt, wd) where fmt is "%a" widened to charT.
Otherwise inserts unsigned{wd} << " is not a valid weekday".
Returns: os.
```template<class charT, class traits> basic_ostream<charT, traits>& to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const weekday& wd); ```
Effects: Streams wd into os using the format specified by the NTCTS fmt.
fmt encoding follows the rules specified in [time.format].
Returns: os.
```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.