code: def iso8601_datetime(d):
      
      if d == values.unset:
          return d
      elif isinstance(d, datetime.datetime) or isinstance(d, datetime.date):
          return d.strftime('%Y-%m-%dT%H:%M:%SZ')
      elif isinstance(d, str):
          return d
docs: Return a string representation of a date that the Twilio API understands
      Format is YYYY-MM-DD. Returns None if d is not a string, datetime, or date