Use `parsedate_tz` in the linkcheck builder

sphinx/builders/linkcheck.py

@@ -6,8 +6,7 @@ import json
 import re
 import socket
 import time
-from datetime import datetime, timezone
-from email.utils import parsedate_to_datetime
+from email.utils import parsedate_tz
 from html.parser import HTMLParser
 from os import path
 from queue import PriorityQueue, Queue
@@ -490,14 +489,15 @@ class HyperlinkAvailabilityCheckWorker(Thread):
             except ValueError:
                 try:
                     # An HTTP-date: time of next attempt.
-                    until = parsedate_to_datetime(retry_after)
+                    parsed = parsedate_tz(retry_after)
+                    # the 10th element is the GMT offset in seconds
+                    next_check = time.mktime(parsed[:9]) - parsed[9]
                 except (TypeError, ValueError):
                     # TypeError: Invalid date format.
                     # ValueError: Invalid date, e.g. Oct 52th.
                     pass
                 else:
-                    next_check = datetime.timestamp(until)
-                    delay = (until - datetime.now(timezone.utc)).total_seconds()
+                    delay = next_check - time.time()
             else:
                 next_check = time.time() + delay
         netloc = urlsplit(response_url).netloc

tests/test_build_linkcheck.py

@@ -9,7 +9,6 @@ import textwrap
 import time
 import wsgiref.handlers
 from base64 import b64encode
-from datetime import datetime
 from os import path
 from queue import Queue
 from unittest import mock
@@ -775,8 +774,7 @@ def test_too_many_requests_retry_after_int_delay(app, capsys, status):
 
 @pytest.mark.sphinx('linkcheck', testroot='linkcheck-localserver', freshenv=True)
 def test_too_many_requests_retry_after_HTTP_date(app, capsys):
-    now = datetime.now().timetuple()
-    retry_after = wsgiref.handlers.format_date_time(time.mktime(now))
+    retry_after = wsgiref.handlers.format_date_time(time.time())
     with http_server(make_retry_after_handler([(429, retry_after), (200, None)])):
         app.build()
     content = (app.outdir / 'output.json').read_text(encoding='utf8')