wxml：

    <text class='marright'>{{hours}}:{{minute}}:{{second}}</text>

js：

page({
    data: {
        hours: '0' + 0,   // 时
        minute: '0' + 0,   // 分
        second: '0' + 0    // 秒
    },
        
    onLoad: function (options) {
        // 调用函数
        this.setInterval()
    }, 
    
    // 计时器
    setInterval: function () {
        const that = this
        var second = that.data.second
        var minute = that.data.minute
        var hours = that.data.hours       
        setInterval(function () {  // 设置定时器
            second++
            if (second >= 60) {
                second = 0  //  大于等于60秒归零
                minute++
                if (minute >= 60) {
                    minute = 0  //  大于等于60分归零
                    hours++
                    if (hours < 10) {
                        // 少于10补零
                        that.setData({
                            hours: '0' + hours
                        })
                    } else {
                        that.setData({
                            hours: hours
                        })
                    }
                }
                if (minute < 10) {
                    // 少于10补零
                    that.setData({
                        minute: '0' + minute
                    })
                } else {
                    that.setData({
                        minute: minute
                    })
                }
            }
            if (second < 10) {
                // 少于10补零
                that.setData({
                    second: '0' + second
                })
            } else {
                that.setData({
                    second: second
                })
            }
        }, 1000)
    },
})