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 <cassert>
 #include <chrono>
 #include <iostream>
 int main()
 {
     constexpr auto monthsInYear{12};
     auto ymd{std::chrono::day(1)/std::chrono::July/2020};
     std::cout << "#1 " << ymd << '\n';
     ymd -= std::chrono::years(10);
     std::cout << "#2 " << ymd << '\n';
     assert(ymd.month() == std::chrono::July);
     assert(ymd.year() == std::chrono::year(2010));
     ymd += std::chrono::months(10 * monthsInYear + 11);
     std::cout << "#3 " << ymd << '\n';
     assert(ymd.month() == std::chrono::month(6));
     assert(ymd.year() == std::chrono::year(2021));
     // Handling the ymd += months "overflow" case.
     ymd = std::chrono::May/31/2021; // ok
     std::cout << "#4 " << ymd << '\n';
     assert(ymd.ok());
     ymd += std::chrono::months{1}; // bad date: June has only 30 days
     std::cout << "#5 " << ymd << '\n';
     assert(not ymd.ok());
     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 << "#6 " << ymd1 << '\n';
     assert(ymd1.ok());
     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 << "#7 " << ymd2 << '\n';
     assert(ymd2.ok());
     assert(ymd2 == std::chrono::July/1/2021);
 }

Output:
 #1 2020-07-01
 #2 2010-07-01
 #3 2021-06-01
 #4 2021-05-31
 #5 2021-06-31 is not a valid date
 #6 2021/Jun/last
 #7 2021-07-01

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