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

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

discord/client.py

@@ -185,6 +185,14 @@ class Client:
 
             In short, this makes it so the only member you can reliably query is the
             message author. Useful for bots that do not require any state.
+    assume_unsync_clock: :class:`bool`
+        Whether to assume the system clock is unsynced. This applies to the ratelimit handling
+        code. If this is set to ``True``, the default, then the library uses the time to reset
+        a rate limit bucket given by Discord. If this is ``False`` then your system clock is
+        used to calculate how long to sleep for. If this is set to ``False`` it is recommended to
+        sync your system clock to Google's NTP server.
+
+        .. versionadded:: 1.3
 
     Attributes
     -----------
@@ -203,7 +211,8 @@ class Client:
         connector = options.pop('connector', None)
         proxy = options.pop('proxy', None)
         proxy_auth = options.pop('proxy_auth', None)
-        self.http = HTTPClient(connector, proxy=proxy, proxy_auth=proxy_auth, loop=self.loop)
+        unsync_clock = options.pop('assume_unsync_clock', True)
+        self.http = HTTPClient(connector, proxy=proxy, proxy_auth=proxy_auth, unsync_clock=unsync_clock, loop=self.loop)
 
         self._handlers = {
             'ready': self._handle_ready

discord/http.py

@@ -86,7 +86,7 @@ class HTTPClient:
     SUCCESS_LOG = '{method} {url} has received {text}'
     REQUEST_LOG = '{method} {url} with {json} has returned {status}'
 
-    def __init__(self, connector=None, *, proxy=None, proxy_auth=None, loop=None):
+    def __init__(self, connector=None, *, proxy=None, proxy_auth=None, loop=None, unsync_clock=True):
         self.loop = asyncio.get_event_loop() if loop is None else loop
         self.connector = connector
         self.__session = None # filled in static_login
@@ -97,6 +97,7 @@ class HTTPClient:
         self.bot_token = False
         self.proxy = proxy
         self.proxy_auth = proxy_auth
+        self.use_clock = not unsync_clock
 
         user_agent = 'DiscordBot (https://github.com/Rapptz/discord.py {0}) Python/{1[0]}.{1[1]} aiohttp/{2}'
         self.user_agent = user_agent.format(__version__, sys.version_info, aiohttp.__version__)
@@ -166,7 +167,7 @@ class HTTPClient:
                     remaining = r.headers.get('X-Ratelimit-Remaining')
                     if remaining == '0' and r.status != 429:
                         # we've depleted our current bucket
-                        delta = utils._parse_ratelimit_header(r)
+                        delta = utils._parse_ratelimit_header(r, use_clock=self.use_clock)
                         log.debug('A rate limit bucket has been exhausted (bucket: %s, retry: %s).', bucket, delta)
                         maybe_lock.defer()
                         self.loop.call_later(delta, lock.release)

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):
+def _parse_ratelimit_header(request, *, use_clock=False):
-    now = parsedate_to_datetime(request.headers['Date'])
+    reset_after = request.headers.get('X-Ratelimit-Reset-After')
-    reset = datetime.datetime.fromtimestamp(float(request.headers['X-Ratelimit-Reset']), datetime.timezone.utc)
+    if use_clock or not reset_after:
-    return (reset - now).total_seconds()
+        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)