std::chrono::operator+, std::chrono::operator- (std::chrono::month)

Since C++20

Header: <chrono>

1,2) Adds ms.count() months to m. The month value held in the result is computed by first evaluating static_cast(unsigned(m)) + (ms.count() - 1), reducing it modulo 12 to an integer in the range [0,11], and then adding 1.

# Declarations

constexpr std::chrono::month operator+( const std::chrono::month& m,
const std::chrono::months& ms ) noexcept;

(since C++20)

constexpr std::chrono::month operator+( const std::chrono::months& ms,
const std::chrono::month& m ) noexcept;

(since C++20)

constexpr std::chrono::month operator-( const std::chrono::month& m,
const std::chrono::months& ms ) noexcept;

(since C++20)

constexpr std::chrono::months operator-( const std::chrono::month& m1,
const std::chrono::month& m2 ) noexcept;

(since C++20)

# Notes

As long as the computation doesn’t overflow, (1-3) always return a valid month even if m.ok() is false.

The result of subtracting two month values is a duration of type std::chrono::months. That duration unit represents the length of the average Gregorian month, and the resulting duration bears no relationship to the number of days in the particular months represented by the operands. For example, std::chrono::seconds(std::chrono::April - std::chrono::March) is not the number of seconds in March (2678400s), but 2629746s (30.436875 days).

# Example

#include <cassert>
#include <chrono>
 
int main()
{
    std::chrono::month m{6};
 
    m = m + std::chrono::months(2);
    assert(m == std::chrono::month(8));
 
    m = m - std::chrono::months(3);
    assert(m == std::chrono::month(5));
 
    constexpr std::chrono::months ms = std::chrono::month(8) - std::chrono::month(6);
    static_assert(ms == std::chrono::months(2));
}

# See also