日時文字列 (yyyyMMddHHmmss) → Date オブジェクト

/**
 * yyyyMMddHHmmss な文字列を Date オブジェクトに変換
 * @param {string} strDate
 */
function stringToDate(strDate) {
    const year = parseInt(strDate.substring(0, 4))
    const month = parseInt(strDate.substring(4, 6))
    const date = parseInt(strDate.substring(6, 8))
    const hour = parseInt(strDate.substring(8, 10))
    const min = parseInt(strDate.substring(10, 12))
    const sec = parseInt(strDate.substring(12, 14))
    return new Date(year, month, date, hour, min, sec)
}

Date オブジェクト→ 日時文字列 (yyyyMMddHHmmss)

/**
 * Date オブジェクトを yyyyMMddHHmmss 形式の文字列に変換
 * @param {Date} date 変換対象の Date オブジェクト
 */
function dateToString(date) {
    const strYear = String(date.getFullYear()).padStart(4, '0')
    const strMonth = String(date.getMonth()).padStart(2, '0')
    const strDate = String(date.getDate()).padStart(2, '0')
    const strHour = String(date.getHours()).padStart(2, '0')
    const strMin = String(date.getMinutes()).padStart(2, '0')
    const strSec = String(date.getSeconds()).padStart(2, '0')
    return strYear + strMonth + strDate + strHour + strMin + strSec
}

※ padStart() は IE 未対応なので注意

参考ページ

String.prototype.substring() - JavaScript | MDN
- https://developer.mozilla.org/ja/docs/Web/JavaScript/Reference/Global_Objects/string/substring
String.prototype.padStart() - JavaScript | MDN
- https://developer.mozilla.org/ja/docs/Web/JavaScript/Reference/Global_Objects/String/padStart
Date - JavaScript | MDN
- https://developer.mozilla.org/ja/docs/Web/JavaScript/Reference/Global_Objects/Date