duration.js

function parseDesignators (duration) {
  return [
    {
      name: 'hours',
      designator: 'H'
    },
    {
      name: 'minutes',
      designator: 'M'
    },
    {
      name: 'seconds',
      designator: 'S'
    }
  ].reduce(
    (acc, item) => {
      const index = duration.indexOf(item.designator)
      if (index !== -1) {
        acc[item.name] = index
      }

      return acc
    },
    {}
  )
}

function parseHours (duration, designators) {
  if (!designators.hours) {
    return '0'
  }

  // always start after the PT
  return duration.substring(2, designators.hours)
}

function parseMinutes(duration, designators) {
  if (!designators.minutes) {
    return '0'
  }

  // start after the hours designator if present
  if (designators.hours) {
    return duration.substring(designators.hours + 1, designators.minutes)
  }

  // always start after the PT if hours is not present
  return duration.substring(2, designators.minutes)
}

function parseSeconds(duration, designators) {
  if (!designators.seconds) {
    return '0'
  }

  // start after the minutes designator if present
  if (designators.minutes) {
    return duration.substring(designators.minutes + 1, designators.seconds)
  }

  // start after the hours designator if present and hours is not present
  if (designators.hours) {
    return duration.substring(designators.hours + 1, designators.seconds)
  }

  // always start after the PT if neither hours or minutes is present
  return duration.substring(2, designators.seconds)
}

function parseDuration (duration) {
  const designators = parseDesignators(duration)
  return {
    hours: Number(parseHours(duration, designators)),
    minutes: Number(parseMinutes(duration, designators)),
    seconds: Number(parseSeconds(duration, designators))
  }
}

function parseDurationToSeconds(duration) {
  const values = parseDuration(duration)
  return (values.hours * 3600) + (values.minutes * 60) + values.seconds
}