Returns %TRUE if the day-month-year triplet forms a valid, existing day in the range of days #GDate understands (Year 1 or later, no more than a few thousand years in the future).
day
month
year
%TRUE if the date is a valid one
See Implementation
Returns %TRUE if the day-month-year triplet forms a valid, existing day in the range of days #GDate understands (Year 1 or later, no more than a few thousand years in the future).