foo.c: In function ‘main’:
foo.c:6:9: error: ‘cin’ undeclared (first use in this function)
    6 |         cin>>year>>month;
      |         ^~~
foo.c:6:9: note: each undeclared identifier is reported only once for each function it appears in
foo.c:28:9: error: ‘cout’ undeclared (first use in this function)
   28 |         cout<<day<<endl;
      |         ^~~~
foo.c:28:20: error: ‘endl’ undeclared (first use in this function)
   28 |         cout<<day<<endl;
      |                    ^~~~