foo.c: In function ‘main’:
foo.c:6:9: warning: implicit declaration of function ‘scanf’ [-Wimplicit-function-declaration]
    6 |         scanf("%d %d", &year, &month);
      |         ^~~~~
foo.c:1:1: note: include ‘<stdio.h>’ or provide a declaration of ‘scanf’
  +++ |+#include <stdio.h>
    1 | int main()
foo.c:6:9: warning: incompatible implicit declaration of built-in function ‘scanf’ [-Wbuiltin-declaration-mismatch]
    6 |         scanf("%d %d", &year, &month);
      |         ^~~~~
foo.c:6:9: note: include ‘<stdio.h>’ or provide a declaration of ‘scanf’
foo.c:7:27: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
    7 |         if (year % 4 == 0 && year % 100 != 0 || (year % 400 == 0))
      |             ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
foo.c:33:9: warning: implicit declaration of function ‘printf’ [-Wimplicit-function-declaration]
   33 |         printf("%d\n", day);
      |         ^~~~~~
foo.c:33:9: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’
foo.c:33:9: warning: incompatible implicit declaration of built-in function ‘printf’ [-Wbuiltin-declaration-mismatch]
foo.c:33:9: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’