jsは2つの日付の時間差の年数、月数、日数、時間数、分、秒数を計算します.(毎月の日数を考慮して特殊処理を行います.)

/**
 *              、  、  、   、   、  
 * DIFFTIME(    ,    ,[  ])，      "y" 、"M"、"d"、"h"、"m"、"s"'
 * console.log(DIFFTIME('2019-6-30 13:20:00', '2020-10-01 11:20:32', 's'))
 */
 
export const DIFFTIME= function (startTime, endTime, unit) {
	//        
    function getDays (mouth, year) {
        let days = 30
        if (mouth === 2) {
            days = year % 4 === 0 ? 29 : 28
        } else if (mouth === 1 || mouth === 3 || mouth === 5 || mouth === 7 || mouth === 8 || mouth === 10 || mouth === 12) {
            //    ：1,3,5,7,8,10,12  ，   .    31；
            days = 31
        }
        return days
    }
    const start = new Date(startTime)
    const end = new Date(endTime)
    //        
    const diffValue = end - start
    //    
    const startYear = start.getFullYear()
    const endYear = end.getFullYear()
    //    
    const startMouth = start.getMonth() + 1
    const endMouth = end.getMonth() + 1
    //    
    const startDay = start.getDate()
    const endDay = end.getDate()
    //     
    const startHours = start.getHours()
    const endHours = end.getHours()
    //    
    const startMinutes = start.getMinutes()
    const endMinutes = end.getMinutes()
    //    
    const startSeconds = start.getSeconds()
    const endSeconds = end.getSeconds()
    //           
    // console.log('start:', startYear, startMouth, startDay, startHours, startMinutes, startSeconds)
    // console.log('end:', endYear, endMouth, endDay, endHours, endMinutes, endSeconds)
    if (unit === 'y' || unit === 'M') {
        //       
        const diffYear = endYear - startYear
        //        
        const startDays = getDays(startMouth, startYear)
        const endDays = getDays(endMouth, endYear)
        const diffStartMouth = (startDays - (startDay + ((startHours * 60 + startMinutes + startSeconds / 60) / 60 / 24) - 1)) / startDays
        const diffEndMouth = (endDay + ((endHours * 60 + endMinutes + endSeconds / 60) / 60 / 24) - 1) / endDays
        const diffMouth = diffStartMouth + diffEndMouth + (12 - startMouth - 1) + endMouth + (diffYear - 1) * 12
        if (unit === 'y') {
            return Math.floor(diffMouth / 12 * 100) / 100
        } else {
            return diffMouth
        }
    } else if (unit === 'd') {
        const d = parseInt(diffValue / 1000 / 60 / 60 / 24)
        return d
    } else if (unit === 'h') {
        const h = parseInt(diffValue / 1000 / 60 / 60)
        return h
    } else if (unit === 'm') {
        const m = parseInt(diffValue / 1000 / 60)
        return m
    } else if (unit === 's') {
        const s = parseInt(diffValue / 1000)
        return s
    } else {
        console.log('        ')
    }
}