Idea for inject embedded countdown DOM element

embedded_countdown.js

(function() {
    const today = new Date()

    const _targetTime = prompt('Input your end time', `${today.getMonth() + 1}/${today.getDate()}/${today.getFullYear()} ${today.getHours()}:${today.getMinutes()}:${today.getSeconds()}`)
    const targetTime = new Date(_targetTime)

    if (isNaN(targetTime)) {
        throw new Error('Your time input is invalid date format.')
    }

    // Create Clock wrapper
    const clockContainer = document.createElement('DIV')
    clockContainer.style = `
        position: fixed;
        top: 24px;
        right: 24px;
        z-index: 1024;
        padding: 18px;
        background: #fff;
    `

    // Create Clock element
    const clockElement = document.createElement('DIV')
    const clockText = document.createElement('SPAN')
    clockText.style = 'font-size: 18pt;'
    const clockTextId = 'clock-countdown-txt'
    clockText.id = clockTextId

    clockElement.appendChild(clockText)
    clockContainer.appendChild(clockElement)
    document.body.appendChild(clockContainer)

    const _clockText = document.querySelector(`#${clockTextId}`)

    // Function
    const formatNumber = num => Number(num) > 9 ? num : '0' + num
    function setTime(hh, mm, ss) {
        _clockText.textContent = `${formatNumber(hh)}:${formatNumber(mm)}:${formatNumber(ss)}`
    }

    // !!! NEED TO APPLY
    function calculateDiff() {
        // return difference time in hour, minute, second
        return { hour: 6, minute: 30, second: 3 }
    }

    function clockUpdate() {
        const { hour, minute, second } = calculateDiff()

        setTime(hour, minute, second)
    }

    clockUpdate()
    setInterval(clockUpdate, 1000)
})()