These problems are found by clang with -Werror enabled.

In src/gtd-application.c, %Id is used in printf format string, but it is a glibc extension and it may be unsupported on other systems.

In src/gtd-task.c, clang finds a possible use of uninitialized variable.

gtd-task.c:638:16: error: variable 'idt' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
      else if (!dt)
               ^~~
gtd-task.c:646:23: note: uninitialized use occurs here
      comp_dt.value = idt;
                      ^~~
gtd-task.c:638:12: note: remove the 'if' if its condition is always true
      else if (!dt)
           ^~~~~~~~
gtd-task.c:606:24: note: initialize the variable 'idt' to silence this warning
      icaltimetype *idt;
                       ^
                        = NULL
1 error generated.