class YachtTimer:
    running = 0
    time_ceil = 0
    time_start = None
    start_from = -300

    def seconds(self):
        if self.running:
            return(self.start_from + time.clock() - self.time_start)
        else:
            return(self.start_from)

    def reset(self, s):
        self.running = 0
        self.start_from = s

    def start(self):
        if not self.running:
            self.running = 1
            self.time_start = time.clock()

    def adjust(self, sec):
        if self.running:
            self.time_start -= sec
        else:
            self.start_from += sec

    def ceil(self):
        if self.running:
            skip = self.seconds()%60
            if time.clock()-self.time_ceil < 1:
                skip += 60
            self.adjust(-skip)
            self.time_ceil = time.clock()
        else:
            self.start_from -= 60

    def floor(self):
        if self.running:
            skip = self.seconds()%60
            self.adjust(60-skip)
        else:
            self.start_from += 60

    def round(self):
        skip = self.seconds()%60
        if skip > 30:
            self.adjust(60-skip)
        else:
            self.adjust(-skip)

This class implements a clock for counting down the starting sequence of a sailing race start. There are methods for adjusting the clock before and after it has been started. Could be used with text, graphic or audio interfaces.

Start clock class

Python