print number of days' validity left for SSL certificate

days_left.py

#!/usr/bin/python3
# vi:ts=4:sw=4:ai:cindent

# reads the a .pem file (or stdin) and emits the number of days'
# validity remaining

import ssl
import OpenSSL.crypto
from datetime import datetime

import sys
with open(sys.argv[1], 'rt') if len(sys.argv) > 1 else sys.stdin as f:
    st_cert = f.read()

c=OpenSSL.crypto
cert=c.load_certificate(c.FILETYPE_PEM, st_cert)

# print(cert.get_subject().CN)

# ASN1 GENERALIZEDTIME:
# YYYYMMDDhhmmssZ
na = cert.get_notAfter()
if(na == None):
    raise Exception("no expiration date")
na = na.decode()
# print(na)
expire_date = datetime.strptime(na, "%Y%m%d%H%M%SZ")
expire_in = (expire_date - datetime.now()).days

if expire_in < 0:
    expire_in = 0
print(expire_in)