惯例，直接上代码：

--正计时开始时的时间戳
self.begin_time = os.time()
--倒计时时长，01:30:00
self.countdown_time = 5400
--是否开始计时
self.is_update_local_time = true

--Unity Update
function time_transition:update_local_timer()
    if not self.is_update_local_time then 
        return
    end
    self.begin_time = self.begin_time + 1
    --正计时
    local positive_time = self.begin_time
    if positive_time > self.countdown_time then
        self.is_update_local_time = false
        self.text_positive_time.text = "01:30:00"
    else
        local positive_time_table = {}
        if positive_time < 60 then
            positive_time_table.hour = 0
            positive_time_table.minute = 0
            positive_time_table.second = positive_time
        elseif positive_time > 60 and positive_time < 3600 then
            positive_time_table.hour = 0
            positive_time_table.minute = math.modf(positive_time / 60)
            positive_time_table.second = positive_time - positive_time_table.minute * 60
        else
            positive_time_table.hour = math.modf(positive_time / 3600)
            positive_time_table.minute = math.modf((positive_time - positive_time_table.hour * 3600) / 60)
            positive_time_table.second = positive_time - positive_time_table.minute * 60 - positive_time_table.hour * 3600
        end
        self.text_positive_time.text = self:local_time_format(positive_time_table)
    end
    --倒计时
    local countdown_time = self.countdown_time - positive_time
    local countdown_time_table = {}
    if countdown_time > 3600 then
        countdown_time_table.hour = math.modf(countdown_time / 3600)
        countdown_time_table.minute = math.modf((countdown_time - countdown_time_table.hour * 3600) / 60)
        countdown_time_table.second = countdown_time - countdown_time_table.minute * 60 - countdown_time_table.hour * 3600
    elseif countdown_time > 60 and countdown_time < 3600 then
        countdown_time_table.hour = 0
        countdown_time_table.minute = math.modf(countdown_time / 60)
        countdown_time_table.second = countdown_time - countdown_time_table.minute * 60
    else
        countdown_time_table.hour = 0
        countdown_time_table.minute = 0
        countdown_time_table.second = countdown_time
    end
    if countdown_time > 0 then
        self.text_countdown_time.text = self:local_time_format(countdown_time_table)
    else
        self.text_countdown_time.text = "00:00:00"
    end
end

--格式化时间:00:00:00
function time_transition:local_time_format(time_table)
	local str_time = ""
	if time_table.hour then
		if time_table.hour >= 0 and time_table.hour < 10 then
			str_time = string.format("0%d" ,time_table.hour)
		else
			str_time = string.format("%d" ,time_table.hour)
		end
	end
	if time_table.minute then
		if time_table.minute >= 0 and time_table.minute < 10 then
			str_time = string.format("%s:0%d" ,str_time ,time_table.minute)
		else
			str_time = string.format("%s:%d" ,str_time ,time_table.minute)
		end
	end
	if time_table.second then
		if time_table.second >= 0 and time_table.second < 10 then
			str_time = string.format("%s:0%d" ,str_time ,time_table.second)
		else
			str_time = string.format("%s:%d" ,str_time ,time_table.second)
		end
	end
	return str_time
end