std::chrono::year_month_day::operator sys_days, std::chrono::year_month_day::operator local_days

constexpr operator std::chrono::sys_days() const noexcept;	(1)	(since C++20)
explicit constexpr operator std::chrono::local_days() const noexcept;	(2)	(since C++20)

Converts *this to a std::chrono::time_point representing the same date as this year_month_day.

1) If ok() is true, the return value holds a count of days from the std::chrono::system_clock epoch (1970-01-01) to *this. The result is negative if *this represent a date prior to it.

Otherwise, if the stored year and month are valid (year().ok() && month().ok() is true), then the returned value is sys_days(year()/month()/1d) + (day() - 1d).

Otherwise (if year().ok() && month().ok() is false), the return value is unspecified.

A sys_days in the range [std::chrono::days{-12687428}, std::chrono::days{11248737}], when converted to year_month_day and back, yields the same value.

2) Same as (1) but returns local_days instead. Equivalent to return local_days(sys_days(*this).time_since_epoch());.

Notes

Converting to sys_days and back can be used to normalize a year_month_day that contains an invalid day but a valid year and month:

using namespace std::chrono;
auto ymd = 2017y/January/0;
ymd = sys_days{ymd};
// ymd is now 2016y/December/31

Normalizing the year and month can be done by adding (or subtracting) zero std::chrono::months:

using namespace std::chrono;
constexpr year_month_day normalize(year_month_day ymd){
    ymd += months{0}; // normalizes year and month
    return sys_days{ymd}; // normalizes day
}
static_assert(normalize(2017y/33/59) == 2019y/10/29);

Example

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