From ae0d97bb260a689c4165febb12f9d716d632112c Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sun, 13 Aug 2023 20:02:28 +0100
Subject: [PATCH] Use ``parsedate_tz`` in the linkcheck builder

---
 sphinx/builders/linkcheck.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/sphinx/builders/linkcheck.py b/sphinx/builders/linkcheck.py
index 1de3fe0eb..4825d51f6 100644
--- a/sphinx/builders/linkcheck.py
+++ b/sphinx/builders/linkcheck.py
@@ -490,9 +490,10 @@ class HyperlinkAvailabilityCheckWorker(Thread):
                 try:
                     # An HTTP-date: time of next attempt.
                     parsed = parsedate_tz(retry_after)
+                    assert parsed is not None
                     # the 10th element is the GMT offset in seconds
-                    next_check = time.mktime(parsed[:9]) - parsed[9]
-                except (TypeError, ValueError):
+                    next_check = time.mktime(parsed[:9]) - (parsed[9] or 0)
+                except (AssertionError, TypeError, ValueError):
                     # TypeError: Invalid date format.
                     # ValueError: Invalid date, e.g. Oct 52th.
                     pass