DateTime :: day - Get days from date
The function returns the day of the month from the date value, i.e. iDays from the iYears-iMonth-iDays date value.
passing values over 31 or smaller than 1 will result in a undefined date
Return value:  Day ( int32  )
Day of the month according to the Gregorian Calendar.
Implementation overview
-  - internal feature (not documented)
 DateTime :: day (  )
-  - internal feature (not documented)
 DateTime :: day ( iDay )

