const dateFromTime = (time) => {
  const [hours, minutes, seconds] = time.split(':').map(Number)
  const date = new Date(null, null, null, hours, minutes, seconds || 0);
  return date;
}

const timeFromDate = (date) => {
  const [hours, minutes] = date.toLocaleTimeString().split(':')
  return `${hours}:${minutes}`
}

const addMinutes = (date, minutes) => {
  return new Date(+date + 1000 * 60 * minutes)
}

const addTime = (timeString, minutesDelta) => timeFromDate(addMinutes(dateFromTime(timeString), minutesDelta))


console.log(addTime('12:20', -7))