/in/foo.c: In function 'main':
/in/foo.c:6:2: error: 'cin' undeclared (first use in this function)
  cin>>year>>month;
  ^~~
/in/foo.c:6:2: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:28:2: error: 'cout' undeclared (first use in this function)
  cout<<day<<endl;
  ^~~~
/in/foo.c:28:13: error: 'endl' undeclared (first use in this function)
  cout<<day<<endl;
             ^~~~