如何在 vu-countdown 中使用变量作为剩余时间？

项目中有个倒计时的需求，之前用 vue-countdown 做过获取验证码的倒计时，但是时间是写死的，现在想再用这个来做，但是把时间设置为变量之后倒计时不会发生变化，代码如下：

<vue-countdown :time="countDown" :interval="1000" :auto-start="true" ref="countdown" class="count-down">
    <template slot-scope="props">{{ props.days }}天{{ props.hours }}时{{ props.minutes }}分{{ props.seconds }}秒</template>
</vue-countdown>

<script>
import axios from 'axios'
import VueCountdown from '@xkeshi/vue-countdown'
export default {
    components: {
        VueCountdown
    },
    data () {
        return {
            countDown: 0
        }
    },
    created: function () {
        axios({
	    method: 'GET',
	    url: 'http://a.b.c',
	    withCredentials: true,
	    headers: {'lang': 'zh', 'token': '', 'os': 'web', 'version': '1.0.0', 'time': ''}
}).then((response) => {
	let responseData = response.data.data
	let responseStatus = response.data.code
	this.needNum = responseData.needNum
	this.timeLeft = responseData.timeLeft
	if (responseStatus == 200) {
		this.payStatus = true
		let now = new Date()
		let timer = (this.timeLeft + 0) * 1000
		let setNow = new Date(now.getTime() + timer)
		this.countDown = setNow - now
		this.$refs.countdown.init()
		this.$refs.countdown.start()
	} else if (responseStatus == 100) {
		this.payStatus = false
	} else if (responseStatus == 101) {

	}
	console.log(responseData)
}).catch((ex) => {
	console.log(ex)
})
    }
}
</script>

在 vue-countdown 的文档里也没看到相关的描述，如果要用它实现的话要怎么做呢？