diff --git a/check_cert_file_expiry.py b/check_cert_file_expiry.py
index 11f8c36..9734680 100755
--- a/check_cert_file_expiry.py
+++ b/check_cert_file_expiry.py
@@ -5,6 +5,7 @@ import datetime
 import argparse
 import enum
 import sys
+from email.utils import parsedate_to_datetime
 
 class Status(enum.Enum):
     OK = 0
@@ -18,12 +19,10 @@ def get_expiry_date(filename):
          "x509",
          "-in", filename,
          "-enddate",
-         "-dateopt", "iso_8601",
          "-noout"],
         text=True, capture_output=True, check=True)
     date = finished_process.stdout.strip().split("=")[1]
-    date = date.replace("Z", "") # needed for python < 3.11, because python is stupid
-    date = datetime.datetime.fromisoformat(date)
+    date = parsedate_to_datetime(date)
     return date
 
 def is_expired(filename, days_from_now=0):
@@ -54,7 +53,8 @@ if __name__ == '__main__':
     try:
         expiry_date = get_expiry_date(args.file)
 
-        perfdata["expires_in"] = (expiry_date - datetime.datetime.now()).days
+
+        perfdata["expires_in"] = (expiry_date - datetime.datetime.now(expiry_date.tzinfo)).days
 
         if is_expired(args.file, 0):
             message = f"Expired on {expiry_date}"