/in/foo.c: In function 'main':
/in/foo.c:18:16: warning: implicit declaration of function 'getDaysInMonth' [-Wimplicit-function-declaration]
   18 |     int days = getDaysInMonth(month, year);
      |                ^~~~~~~~~~~~~~
/usr/bin/ld: /tmp/ccVO50qY.o: in function `main':
foo.c:(.text.startup+0x2f): undefined reference to `getDaysInMonth'
collect2: error: ld returned 1 exit status