こちらを参考にしました。
http://qiita.com/uemura/items/a3a0937f77494e62213c

import して使うことを想定

コマンドから使うと

$ ./cert_expires.py example.com google.com
example.com: 1093 days
google.com: 76 days

sert_expires.py

# !/usr/bin/env python


import sys
import re
import subprocess
import datetime


def parse_result_date_string(date_string):
    """
    convert date string to datetime
    >>> date_string = "Feb  2 05:47:50 2015 GMT"
    >>> d = parse_result_date_string(date_string)
    >>> d
    datetime.datetime(2015, 2, 2, 5, 47, 50)
    """
    d = datetime.datetime.strptime(date_string, '%b %d %H:%M:%S %Y %Z')
    return d


re_start = re.compile(r'Not Before: (.+)')
re_end = re.compile(r'Not After : (.+)')


def get_date_strings_from_result(result):
    """
    :return: cert start date, cert expire date
    >>> text = "Not Before: Feb  2 05:47:47 2015 GMT\\n" \\
    ...        "Not After : Feb  4 16:03:29 2016 GMT"
    >>> get_date_strings_from_result(text)
    ('Feb  2 05:47:47 2015 GMT', 'Feb  4 16:03:29 2016 GMT')
    """
    rr_start = re_start.search(result)
    rr_end = re_end.search(result)
    return rr_start.group(1) if rr_start else None, \
           rr_end.group(1) if rr_end else None


def get_cert_start_expire_date(domain):
    """
    :return: cert start date, cert expire date
    """
    command = "openssl s_client -connect {}:443 < /dev/null " \
              "2> /dev/null | openssl x509 -text | grep Not"

    command = command.format(domain)
    out = subprocess.check_output(command, shell=True)
    ss, es = get_date_strings_from_result(out)
    return parse_result_date_string(ss), parse_result_date_string(es)


def get_cert_expire_delta(domain):
    """
    time delta cert expire date and now
    """
    start_date, expire_date = get_cert_start_expire_date(domain)
    return expire_date - datetime.datetime.now()


def main():
    for domain in sys.argv[1:]:
        delta = get_cert_expire_delta(domain)
        print('{}: {} days'.format(domain, delta.days))


if __name__ == '__main__':
    main()