Extract datetime if available (lazy).

def datetime(self):
        """Extract datetime if available (lazy)."""
        if not self._datetime_calculated:
            self._datetime_calculated = True

            # if no datetime after 10 tokens, break to avoid parsing
            # very long lines
            split_tokens = self.split_tokens[:10]

            for offs in range(len(split_tokens)):
                dt = self._match_datetime_pattern(split_tokens[offs:offs + 4])
                if dt:
                    self._datetime = dt
                    self._datetime_nextpos = offs
                    if self._datetime_format.startswith("iso8601"):
                        self._datetime_nextpos += 1
                    else:
                        self._datetime_nextpos += 4

                    # separate datetime str and linestr
                    self._line_str = (' '.join(self.split_tokens
                                               [self._datetime_nextpos:]))

                    if self.level:
                        self._datetime_nextpos += 2

                    self._reformat_timestamp(self._datetime_format)
                    break

        return self._datetime