updated time

main.go

@@ -11,6 +11,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"time"
 )
 
 //go:embed static/*
@@ -19,6 +20,7 @@ var indexHTML embed.FS
 type store struct {
 	mu    sync.RWMutex
 	value int
+	updatedAt time.Time
 }
 
 func (s *store) get() int {
@@ -30,9 +32,16 @@ func (s *store) get() int {
 func (s *store) set(v int) {
 	s.mu.Lock()
 	s.value = v
+	s.updatedAt = time.Now()
 	s.mu.Unlock()
 }
 
+func (s *store) snapshot() (int, time.Time) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return s.value, s.updatedAt
+}
+
 func main() {
 	secret := os.Getenv("WRITE_SECRET")
 	addr := os.Getenv("ADDR")
@@ -108,7 +117,12 @@ func (s *server) handleIndex(w http.ResponseWriter, r *http.Request) {
 	}
 
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
-	if err := page.Execute(w, map[string]any{"Watts": s.store.get()}); err != nil {
+	watts, updatedAt := s.store.snapshot()
+	updatedLabel := "not updated yet"
+	if !updatedAt.IsZero() {
+		updatedLabel = updatedAt.Format("02.01.2006 15:04:05")
+	}
+	if err := page.Execute(w, map[string]any{"Watts": watts, "UpdatedAt": updatedLabel}); err != nil {
 		http.Error(w, "render error", http.StatusInternalServerError)
 	}
 }

main_test.go

@@ -2,15 +2,16 @@ package main
 
 import (
 	"bytes"
-	"os"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestHandleIndexRendersWatts(t *testing.T) {
-	srv := &server{secret: "secret", store: &store{value: 123}}
+	srv := &server{secret: "secret", store: &store{value: 123, updatedAt: time.Date(2026, 4, 20, 12, 34, 56, 0, time.UTC)}}
 	req := httptest.NewRequest(http.MethodGet, "/", nil)
 	rec := httptest.NewRecorder()
 
@@ -22,6 +23,12 @@ func TestHandleIndexRendersWatts(t *testing.T) {
 	if !strings.Contains(rec.Body.String(), "123") {
 		t.Fatalf("response did not contain watt value: %s", rec.Body.String())
 	}
+	if !strings.Contains(rec.Body.String(), "Last updated:") {
+		t.Fatalf("response did not contain last updated label: %s", rec.Body.String())
+	}
+	if !strings.Contains(rec.Body.String(), "20.04.2026 12:34:56") {
+		t.Fatalf("response did not contain german datetime: %s", rec.Body.String())
+	}
 }
 
 func TestHandleWriteRequiresSecret(t *testing.T) {
@@ -52,6 +59,9 @@ func TestHandleWriteStoresValue(t *testing.T) {
 	if got := srv.store.get(); got != 456 {
 		t.Fatalf("stored watts = %d, want %d", got, 456)
 	}
+	if srv.store.updatedAt.IsZero() {
+		t.Fatal("updatedAt was not set")
+	}
 }
 
 func TestLoadConfig(t *testing.T) {

static/index.html

@@ -11,12 +11,14 @@
     main { text-align: center; padding: 2rem; }
     .value { font-size: clamp(3rem, 12vw, 8rem); font-weight: 800; line-height: 1; }
     .unit { margin-top: 0.75rem; font-size: 1.25rem; color: #94a3b8; }
+    .updated-at { margin-top: 0.5rem; font-size: 0.875rem; color: #64748b; }
   </style>
 </head>
 <body>
-  <main>
-    <div class="value">{{.Watts}}</div>
-    <div class="unit">watts</div>
-  </main>
+    <main>
+      <div class="value">{{.Watts}}</div>
+      <div class="unit">watts</div>
+      <div class="updated-at">Last updated: {{.UpdatedAt}}</div>
+    </main>
 </body>
 </html>