js獲取前幾天的時間及時間區間

/**
 * 以startDate為開始日期，計算時隔days天之后 的日期區間
 * @param startDate 開始日期
 * @param days 時間間隔天數
 */
function getDateInterval(startDate, days) {
    var start;
    var end;
    if (startDate instanceof Date) {
        start = startDate.getFullYear() + "/" + (startDate.getMonth() + 1) + "/" + startDate.getDate();
        var startAfterDays = startDate.setDate(startDate.getDate() - days);
        startAfterDays = new Date(startAfterDays);
        end = startAfterDays.getFullYear() + "/" + (startAfterDays.getMonth() + 1) + "/" + startAfterDays.getDate();
    }
    return end + "~" + start;
}

/**
 * 獲得昨天的日期
 * @returns {Date}
 */
function getYesterdayDate() {
    var date = new Date();
    date.setTime(date.getTime() - 24 * 60 * 60 * 1000);
    return date;
}


/**
 * 根據時間間隔天數，獲取具體日期區間
 * @param days 時間間隔天數
 * @returns {string}
 */
function getStartAndEndDate(days) {
    var now = new Date();
    var start = now.getFullYear() + "/" + (now.getMonth() + 1) + "/" + now.getDate();

    var nowAfterDays = now.setDate(now.getDate() - days);
    nowAfterDays = new Date(nowAfterDays);
    var end = nowAfterDays.getFullYear() + "/" + (nowAfterDays.getMonth() + 1) + "/" + nowAfterDays.getDate();
    return end + "~" + start;
}