Parses ISO 8601 dates into datetime objects
    
    The timezone is parsed from the date string. However it is quite common to
    have dates without a timezone (not strictly correct). In this case the
    default timezone specified in default_timezone is used. This is UTC by
    default.

def parse_date(datestring, default_timezone=UTC):
    """Parses ISO 8601 dates into datetime objects
    
    The timezone is parsed from the date string. However it is quite common to
    have dates without a timezone (not strictly correct). In this case the
    default timezone specified in default_timezone is used. This is UTC by
    default.
    """
    if not isinstance(datestring, basestring):
        raise ParseError("Expecting a string %r" % datestring)
    m = ISO8601_REGEX.match(datestring)
    if not m:
        raise ParseError("Unable to parse date string %r" % datestring)
    groups = m.groupdict()
    tz = parse_timezone(groups["timezone"], default_timezone=default_timezone)
    if groups["fraction"] is None:
        groups["fraction"] = 0
    else:
        groups["fraction"] = int(float("0.%s" % groups["fraction"]) * 1e6)
        
    if groups["hour"] == None and groups["minute"] == None and groups["second"] == None:
        return datetime(int(groups["year"]), int(groups["month"]), int(groups["day"]), tzinfo=tz)
    else:
        return datetime(int(groups["year"]), int(groups["month"]), int(groups["day"]),
            int(groups["hour"]), int(groups["minute"]), int(groups["second"]),
            int(groups["fraction"]), tz)