diff --git a/src/anthropic/_base_client.py b/src/anthropic/_base_client.py
index 852c3b6f..e3a27338 100644
--- a/src/anthropic/_base_client.py
+++ b/src/anthropic/_base_client.py
@@ -3,8 +3,10 @@
 import json
 import time
 import uuid
+import email
 import inspect
 import platform
+import email.utils
 from types import TracebackType
 from random import random
 from typing import (
@@ -616,10 +618,22 @@ def _calculate_retry_timeout(
         try:
             # About the Retry-After header: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Retry-After
             #
-            # TODO: we may want to handle the case where the header is using the http-date syntax: "Retry-After:
             # <http-date>". See https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Retry-After#syntax for
             # details.
-            retry_after = -1 if response_headers is None else int(response_headers.get("retry-after"))
+            if response_headers is not None:
+                retry_header = response_headers.get("retry-after")
+                try:
+                    retry_after = int(retry_header)
+                except Exception:
+                    retry_date_tuple = email.utils.parsedate_tz(retry_header)
+                    if retry_date_tuple is None:
+                        retry_after = -1
+                    else:
+                        retry_date = email.utils.mktime_tz(retry_date_tuple)
+                        retry_after = int(retry_date - time.time())
+            else:
+                retry_after = -1
+
         except Exception:
             retry_after = -1
 
diff --git a/tests/test_client.py b/tests/test_client.py
index fdd1e323..4b38d3ba 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -7,6 +7,7 @@
 import asyncio
 import inspect
 from typing import Any, Dict, Union, cast
+from unittest import mock
 
 import httpx
 import pytest
@@ -423,6 +424,33 @@ class Model(BaseModel):
         response = client.get("/foo", cast_to=Model)
         assert isinstance(response, str)  # type: ignore[unreachable]
 
+    @pytest.mark.parametrize(
+        "remaining_retries,retry_after,timeout",
+        [
+            [3, "20", 20],
+            [3, "0", 2],
+            [3, "-10", 2],
+            [3, "60", 60],
+            [3, "61", 2],
+            [3, "Fri, 29 Sep 2023 16:26:57 GMT", 20],
+            [3, "Fri, 29 Sep 2023 16:26:37 GMT", 2],
+            [3, "Fri, 29 Sep 2023 16:26:27 GMT", 2],
+            [3, "Fri, 29 Sep 2023 16:27:37 GMT", 60],
+            [3, "Fri, 29 Sep 2023 16:27:38 GMT", 2],
+            [3, "99999999999999999999999999999999999", 2],
+            [3, "Zun, 29 Sep 2023 16:26:27 GMT", 2],
+            [3, "", 2],
+        ],
+    )
+    @mock.patch("time.time", mock.MagicMock(return_value=1696004797))
+    def test_parse_retry_after_header(self, remaining_retries: int, retry_after: str, timeout: float) -> None:
+        client = Anthropic(base_url=base_url, api_key=api_key, _strict_response_validation=True)
+
+        headers = httpx.Headers({"retry-after": retry_after})
+        options = FinalRequestOptions(method="get", url="/foo", max_retries=2)
+        calculated = client._calculate_retry_timeout(remaining_retries, options, headers)
+        assert calculated == pytest.approx(timeout, 0.6)  # pyright: ignore[reportUnknownMemberType]
+
 
 class TestAsyncAnthropic:
     client = AsyncAnthropic(base_url=base_url, api_key=api_key, _strict_response_validation=True)
@@ -821,3 +849,31 @@ class Model(BaseModel):
 
         response = await client.get("/foo", cast_to=Model)
         assert isinstance(response, str)  # type: ignore[unreachable]
+
+    @pytest.mark.parametrize(
+        "remaining_retries,retry_after,timeout",
+        [
+            [3, "20", 20],
+            [3, "0", 2],
+            [3, "-10", 2],
+            [3, "60", 60],
+            [3, "61", 2],
+            [3, "Fri, 29 Sep 2023 16:26:57 GMT", 20],
+            [3, "Fri, 29 Sep 2023 16:26:37 GMT", 2],
+            [3, "Fri, 29 Sep 2023 16:26:27 GMT", 2],
+            [3, "Fri, 29 Sep 2023 16:27:37 GMT", 60],
+            [3, "Fri, 29 Sep 2023 16:27:38 GMT", 2],
+            [3, "99999999999999999999999999999999999", 2],
+            [3, "Zun, 29 Sep 2023 16:26:27 GMT", 2],
+            [3, "", 2],
+        ],
+    )
+    @mock.patch("time.time", mock.MagicMock(return_value=1696004797))
+    @pytest.mark.asyncio
+    async def test_parse_retry_after_header(self, remaining_retries: int, retry_after: str, timeout: float) -> None:
+        client = AsyncAnthropic(base_url=base_url, api_key=api_key, _strict_response_validation=True)
+
+        headers = httpx.Headers({"retry-after": retry_after})
+        options = FinalRequestOptions(method="get", url="/foo", max_retries=2)
+        calculated = client._calculate_retry_timeout(remaining_retries, options, headers)
+        assert calculated == pytest.approx(timeout, 0.6)  # pyright: ignore[reportUnknownMemberType]