Sometimes, a signature command could run amuck, and either not produce anything at all or produce things forever. Since xzewd freezes while the command is being run, this would be bad. Therefore, a command timeout is provided. If the signature command is still running after that many seconds have passed, the command is aborted and "(whoops! signature command timed out!)" is used as your signature instead. If your command doesn't work at all, "(whoops! signature command failed!)" is used as your signature.
It's easy to implement random zsigs through the signature command.