'Parses the last-modified value.
For example, last-modified values from a swift object header.
Returns the datetime.datetime of that value.
:param lm: The last-modified value (or None)
:type lm: string
:returns: An offset-naive UTC datetime of the value (or None)'

def parse_last_modified(self, lm):

    if (not lm):
        return None
    pd = email.utils.parsedate(lm)[:6]
    return datetime.datetime(*pd)
