Timer Countdown to control relay (multiple functions in void loop)
I think what you're looking for is to declare your functions near the top of your file, usually before the first function you write, setup(), in this case. So add this just above your setup() function:

// Forward function declarations
void displayNumber(void);
void checkUp(void);
void checkDown(void);
void checkStart(void);
void countDown(void);


C compilers (and thus, C++ because it was derived from C) read the source code only once through, so any function must become known to the compiler ahead of where it is first called. This would require you to place all low-level functions first, then all functions that call those, then the functions that call those, ..., etc. And there are some cases that even this arrangement won't solve, e.g., functions that call each other (oww, my brain hurts!!). So function declarations are used to avoid this inconvenience.

Declaring a function just means informing the compiler of its name, the parameter types, and the return type -- the actual function code isn't required at this point.