def cert_time_to_seconds(cert_time):
    """Return the time in seconds since the Epoch as a floating point
    number, given the cert_time string representing the "notBefore" or
    "notAfter" date from a certificate in "%b %e %H:%M:%S %Y %Z"
    strptime format (C locale).

    >>> timestamp = cert_time_to_seconds("Jan  5 09:34:43 2018 GMT")
    >>> timestamp
    1515144883.0
    >>> from datetime import datetime
    >>> print(datetime.utcfromtimestamp(timestamp))
    2018-01-05 09:34:43

    "notBefore" or "notAfter" dates must use GMT (rfc 3280).

    Month is one of: Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    (see ASN1_TIME_print())

    """
    import re
    from calendar import timegm #NOTE: it brings hardcoded Unix epoch (1970)

    months =  [
        "Jan","Feb","Mar","Apr","May","Jun",
        "Jul","Aug","Sep","Oct","Nov","Dec"
    ]

    # ASN1_TIME_print format: "%s %2d %02d:%02d:%02d %d%s"
    m = re.match("(?i)" # case-insensitive for compatibility with strptime()
                 "(?P<month>{months}) "
                 "(?P<day>(?: |\d)\d) "
                 "(?P<hour>\d\d):"
                 "(?P<minute>\d\d):"
                 "(?P<second>\d\d) "
                 "(?P<year>\d+) GMT".format(months="|".join(months)), cert_time)
    # NOTE: regex is used instead of time.strptime() to avoid
    # dependence on the current locale
    if not m:
        raise ValueError('time data %r does not match '
                         'format %r' % (cert_time, "%b %e %H:%M:%S %Y %Z"))

    month, *rest, year = m.groups()
    #NOTE: avoid datetime constructor to support leap seconds
    # return a float for compatibility (fractional seconds are always zero)
    return float(timegm((int(year), months.index(month.title()) + 1) +
                        tuple(map(int, rest))))

if __name__ =="__main__":
    import doctest; doctest.testmod()