previousCode.js

function convert(line) {
  let timeMatch = /\1(\d\d):\2(\d\d),\3(\d+)\4([sm])/
  if(timeMatch.test(line)) {
    let arr = timeMatch.exec(line)
    let min = Number(arr[1])
    let sec = Number(arr[2])
    let duration = Number(arr[3])
    let time = arr[4]

    let addMin = sec + duration > 59 ? 1 : 0 
    let subSec = addMin === 1 ? 60 : 0

    return {
      startTime: {
        min,
        sec
      },
      endTime: {
        min: min + (time === 'm' ? duration : 0) + addMin,
        sec: sec + (time === 's' ? duration : 0) - subSec
      }
    }
  }
  else return ''
}