This program allows you to generate n-hour clocks with the hand pointing at the f-th hour. I wrote this program to use the images to explain simple modular arithmetic visually.

Clock_creator.pde

void setup(){
  size(350, 350);
  background(255,255,255);
  textSize(26); 
}
void arrow(int x1, int y1, float x2, float y2) {
  line(x1, y1, x2, y2);
  pushMatrix();
  translate(x2, y2);
  float a = atan2(x1-x2, y2-y1);
  rotate(a);
  line(0, 0, -10, -10);
  line(0, 0, 10, -10);
  popMatrix();
}
void draw(){
  int n=12;
  int f=3;
  textAlign(CENTER);
  f=f%n;
  fill(255, 255, 255);
  ellipse(width/2, height/2, 250, 250);
  fill(0, 0, 0);
  ellipse(width/2, height/2, 10, 10);
  for(int i=0; i < n; i++){
    fill(0, 0, 0);
    ellipse(cos(2*PI*i/n-PI/2)*125+width/2, sin(2*PI*i/n-PI/2)*125+height/2, 5, 5);
    text(i, cos(2*PI*i/n-PI/2)*140+width/2, sin(2*PI*i/n-PI/2)*140+height/2+9);
    if(i == f){
      arrow(width/2, height/2, cos(2*PI*i/n-PI/2)*115+width/2, sin(2*PI*i/n-PI/2)*115+height/2);
    } 
  }
  String S="clock_of_"+n+"_hours_at_"+f+".jpg";
  save(S);
}