docs: def parse_now_field(s):
      """Return a datetime instance from a string generated by now_field.
      IMPORTANT: the datetime will be in UTC"""
      if not s.startswith('UTC:'):
          return None  # Invalid string
      s = s[4:]
  
      # isoformat can return strings both with and without microseconds - we
      # account for both
      try:
          dt = datetime.datetime.strptime(s, '%Y-%m-%dT%H:%M:%S.%f')
      except ValueError:
          dt = datetime.datetime.strptime(s, '%Y-%m-%dT%H:%M:%S')
  
      return dt