Use X-Ratelimit-Reset-After header by default.

There is now an option to turn it off, of course.

discord/utils.py

@@ -302,10 +302,15 @@ def _bytes_to_base64_data(data):
 def to_json(obj):
     return json.dumps(obj, separators=(',', ':'), ensure_ascii=True)
 
-def _parse_ratelimit_header(request):
-    now = parsedate_to_datetime(request.headers['Date'])
-    reset = datetime.datetime.fromtimestamp(float(request.headers['X-Ratelimit-Reset']), datetime.timezone.utc)
-    return (reset - now).total_seconds()
+def _parse_ratelimit_header(request, *, use_clock=False):
+    reset_after = request.headers.get('X-Ratelimit-Reset-After')
+    if use_clock or not reset_after:
+        utc = datetime.timezone.utc
+        now = datetime.datetime.now(utc)
+        reset = datetime.datetime.fromtimestamp(float(request.headers['X-Ratelimit-Reset']), utc)
+        return (reset - now).total_seconds()
+    else:
+        return float(reset_after)
 
 async def maybe_coroutine(f, *args, **kwargs):
     value = f(*args, **kwargs)