diff --git a/operation/milestone/milestone.go b/operation/milestone/milestone.go
index b86d701..140059a 100644
--- a/operation/milestone/milestone.go
+++ b/operation/milestone/milestone.go
@@ -177,6 +177,7 @@ func createMilestoneFn(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallT
 	if ok {
 		opt.Description = description
 	}
+	opt.Deadline = params.GetOptionalTime(req.GetArguments(), "due_on")
 
 	client, err := gitea.ClientFromContext(ctx)
 	if err != nil {
@@ -219,6 +220,7 @@ func editMilestoneFn(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToo
 	if ok {
 		opt.State = new(gitea_sdk.StateType(state))
 	}
+	opt.Deadline = params.GetOptionalTime(req.GetArguments(), "due_on")
 
 	client, err := gitea.ClientFromContext(ctx)
 	if err != nil {
diff --git a/operation/milestone/milestone_test.go b/operation/milestone/milestone_test.go
new file mode 100644
index 0000000..7a2a505
--- /dev/null
+++ b/operation/milestone/milestone_test.go
@@ -0,0 +1,84 @@
+package milestone
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"maps"
+	"net/http"
+	"net/http/httptest"
+	"sync"
+	"testing"
+
+	"gitea.com/gitea/gitea-mcp/pkg/flag"
+
+	"github.com/mark3labs/mcp-go/mcp"
+)
+
+func Test_milestoneWriteFn_dueOn(t *testing.T) {
+	const (
+		owner = "octo"
+		repo  = "demo"
+		id    = 42
+		due   = "2026-05-18T23:59:59Z"
+	)
+
+	var (
+		mu     sync.Mutex
+		bodies = map[string]map[string]any{}
+	)
+
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.URL.Path {
+		case "/api/v1/version":
+			_, _ = w.Write([]byte(`{"version":"1.12.0"}`))
+		case fmt.Sprintf("/api/v1/repos/%s/%s/milestones", owner, repo),
+			fmt.Sprintf("/api/v1/repos/%s/%s/milestones/%d", owner, repo, id):
+			mu.Lock()
+			var body map[string]any
+			_ = json.NewDecoder(r.Body).Decode(&body)
+			bodies[r.Method] = body
+			mu.Unlock()
+			_, _ = w.Write(fmt.Appendf(nil, `{"id":%d,"title":"v1","due_on":%q}`, id, due))
+		default:
+			http.NotFound(w, r)
+		}
+	})
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	origHost, origToken, origVersion := flag.Host, flag.Token, flag.Version
+	flag.Host, flag.Token, flag.Version = server.URL, "", "test"
+	defer func() { flag.Host, flag.Token, flag.Version = origHost, origToken, origVersion }()
+
+	args := map[string]any{"owner": owner, "repo": repo, "due_on": due}
+
+	cases := []struct {
+		name   string
+		fn     func(context.Context, mcp.CallToolRequest) (*mcp.CallToolResult, error)
+		method string
+		extra  map[string]any
+	}{
+		{"create", createMilestoneFn, http.MethodPost, map[string]any{"title": "v1"}},
+		{"edit", editMilestoneFn, http.MethodPatch, map[string]any{"id": float64(id)}},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			a := map[string]any{}
+			maps.Copy(a, args)
+			maps.Copy(a, tc.extra)
+			res, err := tc.fn(context.Background(), mcp.CallToolRequest{Params: mcp.CallToolParams{Arguments: a}})
+			if err != nil || res.IsError {
+				t.Fatalf("%s err=%v result=%v", tc.name, err, res)
+			}
+			mu.Lock()
+			body := bodies[tc.method]
+			mu.Unlock()
+			if got, _ := body["due_on"].(string); got != due {
+				t.Fatalf("%s: expected due_on=%q, got %v (body: %v)", tc.name, due, got, body)
+			}
+		})
+	}
+}