def format_time(hh, mm, ss):
    return f"{hh:02d}:{mm:02d}:{ss:02d}"

class Timer:
    def __init__(self, hours, minutes, seconds):
        self.__hours = hours
        self.__minutes = minutes
        self.__seconds = seconds

    def __str__(self):
        return format_time(self.__hours, self.__minutes, self.__seconds)

    def next_second(self):
        self.__seconds += 1
        if self.__seconds > 59:
            self.__seconds = 0
            self.__minutes += 1
        if self.__minutes > 59:
            self.__minutes = 0
            self.__hours += 1
        if self.__hours > 23:
            self.__hours = 0

    def prev_second(self):
        self.__seconds -= 1
        if self.__seconds < 0:
            self.__seconds = 59
            self.__minutes -= 1
        if self.__minutes < 0:
            self.__minutes = 59
            self.__hours -= 1
        if self.__hours < 0:
            self.__hours = 23


timer = Timer(23, 59, 59)
print(timer)
timer.next_second()
print(timer)
timer.prev_second()
print(timer)