docs: def get_duration(timestr1, timestr2, units='s'):
      """
      Return the duration of the time period between the first and second
      time string in the given units.
      Allowable units are 's' (seconds), 'min' (minutes), 'h' (hours).
      Assume ``timestr1 < timestr2``.
      """
      valid_units = ['s', 'min', 'h']
      assert units in valid_units,\
        "Units must be one of {!s}".format(valid_units)
  
      duration = (
          gt.timestr_to_seconds(timestr2) - gt.timestr_to_seconds(timestr1)
      )
  
      if units == 's':
          return duration
      elif units == 'min':
          return duration/60
      else:
          return duration/3600