std::chrono::year_month_day::operator+=,std::chrono::year_month_day::operator-=(3)

NAME
std::chrono::year_month_day::operator+=,std::chrono::year_month_day::operator-= - std::chrono::year_month_day::operator+=,std::chrono::year_month_day::operator-=

Synopsis
   constexpr std::chrono::year_month_day& (1) (since C++20)
   operator+=( const std::chrono::years& dy ) const noexcept;
   constexpr std::chrono::year_month_day& (2) (since C++20)
   operator+=( const std::chrono::months& dm ) const noexcept;
   constexpr std::chrono::year_month_day& (3) (since C++20)
   operator-=( const std::chrono::years& dy ) const noexcept;
   constexpr std::chrono::year_month_day& (4) (since C++20)
   operator-=( const std::chrono::months& dm ) const noexcept;
   Modifies the time point *this represents by the duration dy or dm.
   1) Equivalent to *this = *this + dy;
   2) Equivalent to *this = *this + dm;
   3) Equivalent to *this = *this - dy;
   4) Equivalent to *this = *this - dm;
   For durations that are convertible to both std::chrono::years and
   std::chrono::months, the years overloads (1,3) are preferred if the call would
   otherwise be ambiguous.

Example
// Run this code
 #include <iostream>
 #include <cassert>
 #include <chrono>
 int main()
 {
     std::cout << std::boolalpha;
     constexpr auto monthsInYear {12};
     auto ymd {std::chrono::day(1)/std::chrono::July/2020};
     ymd -= std::chrono::years(10);
     std::cout << (ymd.month() == std::chrono::July) << ' ' // true
               << (ymd.year() == std::chrono::year(2010)) << ' '; // true
     ymd += std::chrono::months(10 * monthsInYear + 11);
     std::cout << (ymd.month() == std::chrono::month(6)) << ' ' // true
               << (ymd.year() == std::chrono::year(2021)) << ' '; // true
     // Handling the ymd += months "overflow" case.
     ymd = std::chrono::May/31/2021; // ok
     std::cout << ymd.ok() << ' '; // true
     ymd += std::chrono::months{1}; // bad date: June has only 30 days
     std::cout << ymd.ok() << ' '; // false
     assert(ymd == std::chrono::June/31/2021);
     // Snap to the last day of the month, June 30:
     const auto ymd1 = ymd.year()/ymd.month()/std::chrono::last;
     std::cout << ymd1.ok() << ' '; // true
     assert(ymd1 == std::chrono::June/30/2021);
     // Overflow into the next month, July 1 (via converting to/from sys_days):
     const std::chrono::year_month_day ymd2 = std::chrono::sys_days{ymd};
     std::cout << ymd2.ok() << ' '; // true
     assert(ymd2 == std::chrono::July/1/2021);
 }

Output:
 true true true true true false true true

See also
   operator+ adds or subtracts a year_month_day and some number of years or months
   operator- (function)
   (C++20)