/in/foo.cc: In member function 'int Date::getFirstDay(const Date&) const':
/in/foo.cc:30:43: error: passing 'const Date' as 'this' argument discards qualifiers [-fpermissive]
   int mon[13]={0,31,28+runnian(date.getY()),31,30,31,30,31,31,30,31,30,31};
                                           ^
/in/foo.cc:43:7: note:   in call to 'bool Date::runnian(int)'
  bool runnian(int y)
       ^~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:60:15: error: no match for 'operator-' (operand types are 'Date' and 'Date')
  int day=date2-date1;
          ~~~~~^~~~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_iterator.h:333:5: note: candidate: template<class _Iterator> decltype ((__x.base() - __y.base())) std::operator-(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
     operator-(const reverse_iterator<_Iterator>& __x,
     ^~~~~~~~
/usr/include/c++/6/bits/stl_iterator.h:333:5: note:   template argument deduction/substitution failed:
/in/foo.cc:60:16: note:   'Date' is not derived from 'const std::reverse_iterator<_Iterator>'
  int day=date2-date1;
                ^~~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_iterator.h:387:5: note: candidate: template<class _IteratorL, class _IteratorR> decltype ((__y.base() - __x.base())) std::operator-(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_IteratorR>&)
     operator-(const reverse_iterator<_IteratorL>& __x,
     ^~~~~~~~
/usr/include/c++/6/bits/stl_iterator.h:387:5: note:   template argument deduction/substitution failed:
/in/foo.cc:60:16: note:   'Date' is not derived from 'const std::reverse_iterator<_Iterator>'
  int day=date2-date1;
                ^~~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_iterator.h:1186:5: note: candidate: template<class _IteratorL, class _IteratorR> decltype ((__x.base() - __y.base())) std::operator-(const std::move_iterator<_IteratorL>&, const std::move_iterator<_IteratorR>&)
     operator-(const move_iterator<_IteratorL>& __x,
     ^~~~~~~~
/usr/include/c++/6/bits/stl_iterator.h:1186:5: note:   template argument deduction/substitution failed:
/in/foo.cc:60:16: note:   'Date' is not derived from 'const std::move_iterator<_IteratorL>'
  int day=date2-date1;
                ^~~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_iterator.h:1193:5: note: candidate: template<class _Iterator> decltype ((__x.base() - __y.base())) std::operator-(const std::move_iterator<_IteratorL>&, const std::move_iterator<_IteratorL>&)
     operator-(const move_iterator<_Iterator>& __x,
     ^~~~~~~~
/usr/include/c++/6/bits/stl_iterator.h:1193:5: note:   template argument deduction/substitution failed:
/in/foo.cc:60:16: note:   'Date' is not derived from 'const std::move_iterator<_IteratorL>'
  int day=date2-date1;
                ^~~~~