def iso_datetime(timestamp=None):
if timestamp is None:<EOL><INDENT>timestamp = time.time()<EOL><DEDENT>return datetime.datetime.fromtimestamp(timestamp).isoformat('<STR_LIT:U+0020>')[:<NUM_LIT>]<EOL>
Convert UNIX timestamp to ISO datetime string.

        @param timestamp: UNIX epoch value (default: the current time).
        @return: Timestamp formatted as "YYYY-mm-dd HH:MM:SS".