def isoformat(sep='T'):
    """Return a string representing the date and time in ISO 8601 format.

        YYYY-MM-DDTHH:MM:SS.mmmmmm or YYYY-MM-DDTHH:MM:SS if microsecond is 0

        If utcoffset() does not return None, a 6-character string is appended,
        giving the UTC offset in (signed) hours and minutes:

        YYYY-MM-DDTHH:MM:SS.mmmmmm+HH:MM or YYYY-MM-DDTHH:MM:SS+HH:MM
        if microsecond is 0.

        The optional argument sep (default 'T') is a one-character separator,
        placed between the date and time portions of the result.
        """
