The current specification of the year_month_day conversion function to sys_days, uses the day member function on the sys_days (a.k.a. time_point<system_clock, days>), that does not exist.

In [time.cal.ymd.members] p18, the expression sys_days{y_/m_/last}.day() is ill-formed:

[…] Otherwise, if y_.ok() && m_.ok() is true, returns a sys_days which is offset from sys_days{y_/m_/last} by the number of days d_ is offset from sys_days{y_/m_/last}.day(). Otherwise the value returned is unspecified.