diff --git a/acme_nginx/Acme.py b/acme_nginx/Acme.py
index b2ae6d1..5ce38be 100644
--- a/acme_nginx/Acme.py
+++ b/acme_nginx/Acme.py
@@ -10,6 +10,7 @@
 import sys
 import tempfile
 import time
+from datetime import datetime, timedelta
 
 try:
     from urllib.request import urlopen, Request  # Python 3
@@ -32,6 +33,7 @@ def __init__(
             cert_path='/etc/ssl/private/letsencrypt-domain.pem',
             dns_provider=None,
             skip_nginx_reload=False,
+            renew_days=None,
             debug=False):
         """
         Params:
@@ -60,6 +62,30 @@ def __init__(
         self.chain = "https://letsencrypt.org/certs/lets-encrypt-x3-cross-signed.pem"
         self.dns_provider = dns_provider
         self.skip_nginx_reload = skip_nginx_reload
+        self.renew_days = renew_days
+        
+        self.IsOutOfDate = True
+        if self.renew_days:
+            try:
+                cert = OpenSSL.crypto.load_certificate(OpenSSL.crypto.FILETYPE_PEM, open(self.cert_path).read())
+                date_format, encoding = "%Y%m%d%H%M%SZ", "ascii"
+                not_before = datetime.strptime(cert.get_notBefore().decode(encoding), date_format)
+                not_after = datetime.strptime(cert.get_notAfter().decode(encoding), date_format)
+                now = datetime.now()
+                #self.log.info( 'x509: {0} {1} {2}'.format(cert, not_before, not_after) )
+                #certTime = datetime.fromtimestamp(os.path.getmtime(self.cert_path))
+                #certTimeThreshold = certTime + timedelta(days=self.renew_days)
+                certTimeThreshold = not_after - timedelta(days=self.renew_days)
+
+                self.IsOutOfDate = (not_before > now) or (not_after < now) or (certTimeThreshold < now)
+                self.log.info('Cert file {1} (expiration time {0})'.format( certTimeThreshold, "is out of date" if self.IsOutOfDate else "is not out of date"))      
+                
+            except OSError as e:
+                if e.errno == 2:
+                    self.log.info('Cert file {0} not found -> DO UPDATE CERT'.format(self.cert_path)) 
+            except:   
+                pass
+            
 
     def _reload_nginx(self):
         """ Reload nginx """
diff --git a/acme_nginx/client.py b/acme_nginx/client.py
index 7750795..188998f 100644
--- a/acme_nginx/client.py
+++ b/acme_nginx/client.py
@@ -74,6 +74,11 @@ def set_arguments():
             dest='skip_reload',
             action='store_true',
             help="don't reload nginx after certificate signing")
+    parser.add_argument(
+            '--renew-days',
+            dest='renew_days',
+            type=int,
+            help="expiration threshold in days")
     return parser.parse_args()
 
 
@@ -107,6 +112,8 @@ def main():
         cert_path=args.cert_path,
         debug=args.debug,
         dns_provider=args.dns_provider,
-        skip_nginx_reload=args.skip_reload
+        skip_nginx_reload=args.skip_reload,
+        renew_days=args.renew_days
     )
-    acme.get_certificate()
+    if acme.IsOutOfDate:
+        acme.get_certificate()