timer.coffee

t = new TextLayer
    text: "00:00"
    style:
        # This style ensures that tabular figures are used, so that all
        # digits are the same width
        fontFeatureSettings: "'tnum'"

# Get the current time
origin = Date.now()

time = ->
    # Get the time at the moment the function is called
    now = Date.now()

    # Get the difference between the 'origin' time and the time right now,
    # then divide by 1000 to turn that from milliseconds to seconds
    secs = Math.floor((now - origin) / 1000)

    # Now, make our seconds run from 0 to 60 and then reset using modulo
    s = secs % 60

    # Divide the seconds by 60 to find the number of minutes that have passed
    min = Math.floor(secs / 60)

    # Add leading zeros if the number of seconds or minutes is less than 10
    if s < 10 then s = "0" + s
    if min < 10 then min = "0" + min

    # Update the text layer
    t.text = "#{min}:#{s}"

# setInterval is JS's native `Utils.interval`, that I used just out of habit.
# It takes a function as its first argument, and N number of milliseconds as
# its second, repeating the function every N milliseconds
timer = setInterval(time, 100)

# If you wanted to stop the timer, you could call this function
# e.g. when `secs` in the `time` function is equal to 100.
stopTimer = ->
    clearInterval(timer)