diff --git a/mitmproxy/net/http/cookies.py b/mitmproxy/net/http/cookies.py
index 1472ab557..2745701f3 100644
--- a/mitmproxy/net/http/cookies.py
+++ b/mitmproxy/net/http/cookies.py
@@ -304,7 +304,7 @@ def refresh_set_cookie_header(c: str, delta: int) -> str:
             e = email.utils.parsedate_tz(attrs["expires"])
             if e:
                 f = email.utils.mktime_tz(e) + delta
-                attrs.set_all("expires", [email.utils.formatdate(f)])
+                attrs.set_all("expires", [email.utils.formatdate(f, usegmt=True)])
             else:
                 # This can happen when the expires tag is invalid.
                 # reddit.com sends a an expires tag like this: "Thu, 31 Dec
diff --git a/test/mitmproxy/net/http/test_response.py b/test/mitmproxy/net/http/test_response.py
index f3470384b..27c16be69 100644
--- a/test/mitmproxy/net/http/test_response.py
+++ b/test/mitmproxy/net/http/test_response.py
@@ -148,7 +148,7 @@ class TestResponseUtils:
     def test_refresh(self):
         r = tresp()
         n = time.time()
-        r.headers["date"] = email.utils.formatdate(n)
+        r.headers["date"] = email.utils.formatdate(n, usegmt=True)
         pre = r.headers["date"]
         r.refresh(946681202)
         assert pre == r.headers["date"]