Decode a JSON datetime to python datetime.datetime.

def _parse_canonical_datetime(doc, json_options):
    """Decode a JSON datetime to python datetime.datetime."""
    dtm = doc["$date"]
    if len(doc) != 1:
        raise TypeError('Bad $date, extra field(s): %s' % (doc,))
    # mongoexport 2.6 and newer
    if isinstance(dtm, string_type):
        # Parse offset
        if dtm[-1] == 'Z':
            dt = dtm[:-1]
            offset = 'Z'
        elif dtm[-3] == ':':
            # (+|-)HH:MM
            dt = dtm[:-6]
            offset = dtm[-6:]
        elif dtm[-5] in ('+', '-'):
            # (+|-)HHMM
            dt = dtm[:-5]
            offset = dtm[-5:]
        elif dtm[-3] in ('+', '-'):
            # (+|-)HH
            dt = dtm[:-3]
            offset = dtm[-3:]
        else:
            dt = dtm
            offset = ''

        # Parse the optional factional seconds portion.
        dot_index = dt.rfind('.')
        microsecond = 0
        if dot_index != -1:
            microsecond = int(float(dt[dot_index:]) * 1000000)
            dt = dt[:dot_index]

        aware = datetime.datetime.strptime(
            dt, "%Y-%m-%dT%H:%M:%S").replace(microsecond=microsecond,
                                             tzinfo=utc)

        if offset and offset != 'Z':
            if len(offset) == 6:
                hours, minutes = offset[1:].split(':')
                secs = (int(hours) * 3600 + int(minutes) * 60)
            elif len(offset) == 5:
                secs = (int(offset[1:3]) * 3600 + int(offset[3:]) * 60)
            elif len(offset) == 3:
                secs = int(offset[1:3]) * 3600
            if offset[0] == "-":
                secs *= -1
            aware = aware - datetime.timedelta(seconds=secs)

        if json_options.tz_aware:
            if json_options.tzinfo:
                aware = aware.astimezone(json_options.tzinfo)
            return aware
        else:
            return aware.replace(tzinfo=None)
    return bson._millis_to_datetime(int(dtm), json_options)