docs: def exact_cftime_datetime_difference(a, b):
      """Exact computation of b - a
  
      Assumes:
  
          a = a_0 + a_m
          b = b_0 + b_m
  
      Here a_0, and b_0 represent the input dates rounded
      down to the nearest second, and a_m, and b_m represent
      the remaining microseconds associated with date a and
      date b.
  
      We can then express the value of b - a as:
  
          b - a = (b_0 + b_m) - (a_0 + a_m) = b_0 - a_0 + b_m - a_m
  
      By construction, we know that b_0 - a_0 must be a round number
      of seconds.  Therefore we can take the result of b_0 - a_0 using
      ordinary cftime.datetime arithmetic and round to the nearest
      second.  b_m - a_m is the remainder, in microseconds, and we
      can simply add this to the rounded timedelta.
  
      Parameters
      ----------
      a : cftime.datetime
          Input datetime
      b : cftime.datetime
          Input datetime
  
      Returns
      -------
      datetime.timedelta
      """
      seconds = b.replace(microsecond=0) - a.replace(microsecond=0)
      seconds = int(round(seconds.total_seconds()))
      microseconds = b.microsecond - a.microsecond
      return datetime.timedelta(seconds=seconds, microseconds=microseconds)