diff --git a/server/upstream/header.go b/server/upstream/header.go
index 9575a3f..7b85df1 100644
--- a/server/upstream/header.go
+++ b/server/upstream/header.go
@@ -24,5 +24,5 @@ func (o *Options) setHeader(ctx *context.Context, header http.Header) {
 	} else {
 		ctx.RespWriter.Header().Set(gitea.ContentTypeHeader, mime)
 	}
-	ctx.RespWriter.Header().Set(headerLastModified, o.BranchTimestamp.In(time.UTC).Format(time.RFC1123))
+	ctx.RespWriter.Header().Set(headerLastModified, o.BranchTimestamp.In(time.UTC).Format(http.TimeFormat))
 }