In file included from main.cpp:16:0:
date.h: In member function 'std::__cxx11::string Date::ToString() const':
date.h:27:21: error: 'to_string' was not declared in this scope
     std::string y = to_string(_year);
                     ^~~~~~~~~
date.h:27:21: note: suggested alternatives:
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from main.cpp:1:
/usr/include/c++/7/bits/basic_string.h:6466:3: note:   'std::__cxx11::to_string'
   to_string(long double __val)
   ^~~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6466:3: note:   'std::__cxx11::to_string'