init python:
    import time

    # The wall time at which the timer should expire.
    timer_end_time = None

    # The place where we should jump to when the timer runs out.
    timer_target = None

    def timer_overlay():
        if timer_end_time is not None:
            ui.timer(timer_end_time - time.time(), ui.jumps(timer_target))

    config.overlay_functions.append(timer_overlay)

    def timer_start(duration, label):
        global timer_end_time
        global timer_jump

        timer_end_time = time.time() + duration
        timer_jump = label

    def timer_end():
        global timer_end_time
        timer_end_time = None

# Use this like:

label event:
    $ timer_start(10, "time_up")

    "Code here."

    $ timer_end()
    jump rest_of_game

label time_up:
    "Time's up!"
    jump rest_of_game