createCanvas(400, 400);

  angleMode(DEGREES); // use degrees rather than radians

  strokeWeight(10)

  stroke('red')

  strokeCap(SQUARE)

  noFill()

}

​

function draw() {

  background(220);

​

  // define the hour we're drawing an arc for. In your code

  // you should just use now.hour (or now.min or now.sec)

  // but in this example try resetting this value by hand

  var hour = 8

  // geometrically 0° is considered to mean 'the right side'

  // rather than 'the top' of a circle, so we'll work in the

  // -90 to 270° range rather than 0 to 360°

  var minAngle = -90

  var maxAngle = 270

​

  // use map to convert our 'hour' variable from the range of 

  // hour values (noon to midnight) into the corresponding 

  // range of angle values defined above

  var hourAngle = map(hour, 0, 12, minAngle, maxAngle)

​

  // draw an arc from the top of the circle (minAngle)

  // to the angle we've calculated for the current hour

  var size = 300

  arc(width/2, height/2, size, size, minAngle, hourAngle)

}