c++ refactorization of the original C language timediff

notes
--------
2019-06-18 fixed bug in ModernDate::isLeftDateSmaller(ModernDate & date1, ModernDate & date2)
    was
        if(date2.dayOfMonth > date2.dayOfMonth) return false; // bug, both are date2
	
    now
        if(date2.dayOfMonth < date1.dayOfMonth) return false; // fixed