diff --git a/src/renderer/App.tsx b/src/renderer/App.tsx
index 430db28..137c22c 100644
--- a/src/renderer/App.tsx
+++ b/src/renderer/App.tsx
@@ -1534,6 +1534,21 @@ export function buildDailyScheduleSettingsUpdate(
   };
 }
 
+export async function activateDailyScheduleSettings(
+  time: string,
+  startDay: DailyScheduleStartDay,
+  persist: (update: RendererSettingsUpdate) => Promise<boolean>,
+  showError: (message: string) => void,
+  now = new Date()
+): Promise<boolean> {
+  const update = buildDailyScheduleSettingsUpdate(time, startDay, now);
+  if (!update) {
+    showError("Bitte eine gültige Startzeit auswählen.");
+    return false;
+  }
+  return persist(update);
+}
+
 export async function persistDailyScheduleSettingsUpdate(
   update: RendererSettingsUpdate,
   operation: "activate" | "cancel",
@@ -4771,14 +4786,17 @@ export function App(): ReactElement {
   ), [applyAuthoritativeDailyScheduleSnapshot, applyDailyScheduleSettings, showToast]);
 
   const activateDownloadSchedule = useCallback((): void => {
-    const update = buildDailyScheduleSettingsUpdate(scheduleTimeInput, scheduleStartDay);
-    if (!update) return;
-    void persistDownloadSchedule(update, "activate").then((persisted) => {
+    void activateDailyScheduleSettings(
+      scheduleTimeInput,
+      scheduleStartDay,
+      (update) => persistDownloadSchedule(update, "activate"),
+      (message) => showToast(message, 2800)
+    ).then((persisted) => {
       if (persisted) {
         setSchedulePickerOpen(false);
       }
     });
-  }, [persistDownloadSchedule, scheduleStartDay, scheduleTimeInput]);
+  }, [persistDownloadSchedule, scheduleStartDay, scheduleTimeInput, showToast]);
 
   const removeActionableDownloads = useCallback((): void => {
     const ids = new Set(downloadsViewCore.actionableSelectedIds);
@@ -4812,6 +4830,7 @@ export function App(): ReactElement {
     scheduleActive: snapshot.settings.dailyStartEnabled || snapshot.settings.scheduledStartEpochMs > 0,
     scheduleOpen: schedulePickerOpen,
     scheduleTime: scheduleTimeInput,
+    scheduleTimeValid: buildDailyScheduleSettingsUpdate(scheduleTimeInput, scheduleStartDay) !== null,
     scheduleStartDay,
     scheduleLabel: scheduleCountdown || (snapshot.settings.dailyStartEnabled
       ? formatDailyScheduleTime(snapshot.settings.dailyStartMinuteOfDay)
diff --git a/src/renderer/i18n.ts b/src/renderer/i18n.ts
index 973569a..2e0a312 100644
--- a/src/renderer/i18n.ts
+++ b/src/renderer/i18n.ts
@@ -63,7 +63,7 @@ const pairs = [
   ["Abgeschlossene und gelöschte Pakete erscheinen hier.", "Completed and deleted packages appear here."], ["Passe Filter oder Suche an.", "Adjust the filter or search."], ["Öffne die Ansicht erneut, um es noch einmal zu versuchen.", "Open the view again to retry."],
   ["Alle sichtbaren Einträge auswählen", "Select all visible entries"], ["Details anzeigen", "Show details"], ["Details ausblenden", "Hide details"],
   ["Sichtbar:", "Visible:"], ["pro Seite", "per page"],
-  ["Verfügbarkeit", "Availability"], ["Hinzugefügt am", "Added on"], ["Ungeprüft", "Unchecked"], ["Paket gestoppt", "Package stopped"], ["Alle anzeigen", "Show all"], ["Planen", "Schedule"], ["Startzeit", "Start time"], ["Starttag", "Start day"], ["Ab heute", "Starting today"], ["Ab morgen", "Starting tomorrow"],
+  ["Verfügbarkeit", "Availability"], ["Hinzugefügt am", "Added on"], ["Ungeprüft", "Unchecked"], ["Paket gestoppt", "Package stopped"], ["Alle anzeigen", "Show all"], ["Planen", "Schedule"], ["Startzeit", "Start time"], ["Starttag", "Start day"], ["Ab heute", "Starting today"], ["Ab morgen", "Starting tomorrow"], ["Bitte eine gültige Startzeit auswählen.", "Select a valid start time."],
   ["Keine Downloads", "No downloads"], ["Keine passenden Downloads", "No matching downloads"], ["Füge Links hinzu, um Downloads vorzubereiten.", "Add links to prepare downloads."], ["Passe Filter oder Suche an.", "Adjust the filter or search."],
   ["Keine Links gesammelt", "No links collected"], ["Keine passenden Links", "No matching links"], ["Füge Links oder Text ein, um sie zu sammeln.", "Paste links or text to collect them."], ["Links durchsuchen", "Search links"],
   ["Datenmenge", "Data volume"], ["Sitzungszähler", "Session counter"], ["Sieben Tage", "Seven days"], ["30 Tage", "30 days"], ["Zeitraum", "Period"], ["Erfolgreich", "Successful"],
diff --git a/src/renderer/views/downloads/DownloadsView.tsx b/src/renderer/views/downloads/DownloadsView.tsx
index 870c86a..0a28093 100644
--- a/src/renderer/views/downloads/DownloadsView.tsx
+++ b/src/renderer/views/downloads/DownloadsView.tsx
@@ -39,9 +39,10 @@ export interface DownloadsViewModel extends DownloadsViewModelCore {
   reconnectSeconds: number;
   reconnectReason: string;
   clipboardWatcher: boolean;
-  scheduleActive: boolean;
+  scheduleActive: boolean;
   scheduleOpen: boolean;
   scheduleTime: string;
+  scheduleTimeValid: boolean;
   scheduleStartDay: DailyScheduleStartDay;
   scheduleLabel: string;
   packageSpeedBps: Record<string, number>;
@@ -141,7 +142,7 @@ export function DownloadsToolbar({ actions, model }: { actions: DownloadsViewAct
         <span {...(!scheduleSlotOpen ? { inert: "true" } : {})} aria-hidden={!scheduleSlotOpen} className="downloads-schedule-controls">
           {model.scheduleActive
             ? <><strong>Geplant: {model.scheduleLabel}</strong><button disabled={false} onClick={actions.onCancelSchedule} type="button">Abbrechen</button></>
-            : <><input aria-label="Startzeit" disabled={!scheduleSlotOpen} onChange={(event) => actions.onScheduleTimeChange(event.target.value)} type="time" value={model.scheduleTime} /><select aria-label="Starttag" disabled={!scheduleSlotOpen} onChange={(event) => actions.onScheduleStartDayChange(event.target.value as DailyScheduleStartDay)} value={model.scheduleStartDay}><option value="today">Ab heute</option><option value="tomorrow">Ab morgen</option></select><button disabled={!scheduleSlotOpen} onClick={actions.onActivateSchedule} type="button">Planen</button></>}
+            : <><input aria-label="Startzeit" disabled={!scheduleSlotOpen} onChange={(event) => actions.onScheduleTimeChange(event.target.value)} type="time" value={model.scheduleTime} /><select aria-label="Starttag" disabled={!scheduleSlotOpen} onChange={(event) => actions.onScheduleStartDayChange(event.target.value as DailyScheduleStartDay)} value={model.scheduleStartDay}><option value="today">Ab heute</option><option value="tomorrow">Ab morgen</option></select><button disabled={!scheduleSlotOpen || !model.scheduleTimeValid} onClick={actions.onActivateSchedule} type="button">Planen</button></>}
         </span>
       </span>
       <span className="downloads-toolbar-divider" />
diff --git a/tests/downloads-view.test.tsx b/tests/downloads-view.test.tsx
index 08324e3..396f240 100644
--- a/tests/downloads-view.test.tsx
+++ b/tests/downloads-view.test.tsx
@@ -791,6 +791,7 @@ function withRuntime(input: DownloadsModelInput, overrides: Partial<DownloadsVie
     scheduleActive: false,
     scheduleOpen: false,
     scheduleTime: "23:30",
+    scheduleTimeValid: true,
     scheduleStartDay: "today",
     scheduleLabel: "",
     packageSpeedBps: { "package-a": 12_000_000 },
@@ -1235,6 +1236,15 @@ describe("downloads view", () => {
     expect(calls).toEqual(["time:09:45", "day:today"]);
   });
 
+  it.each(["", "8:15", "24:00", "12:60"])("disables schedule activation for the invalid time %j", (scheduleTime) => {
+    const toolbar = DownloadsToolbar({
+      actions: createActions(),
+      model: withRuntime(createInput(), { scheduleOpen: true, scheduleTime, scheduleTimeValid: false })
+    });
+
+    expect(findButton(toolbar, "Planen").props.disabled).toBe(true);
+  });
+
   it("animates the persistent schedule slot horizontally and disables it through the global motion setting", () => {
     const closed = renderToStaticMarkup(<DownloadsToolbar actions={createActions()} model={withRuntime(createInput())} />);
     const open = renderToStaticMarkup(<DownloadsToolbar actions={createActions()} model={withRuntime(createInput(), { scheduleOpen: true })} />);
diff --git a/tests/renderer-settings-form.test.tsx b/tests/renderer-settings-form.test.tsx
index 2830e6b..8e91012 100644
--- a/tests/renderer-settings-form.test.tsx
+++ b/tests/renderer-settings-form.test.tsx
@@ -1,6 +1,7 @@
 import { describe, expect, it, vi } from "vitest";
 import type { RendererSettings, UiSnapshot } from "../src/shared/types";
 import {
+  activateDailyScheduleSettings,
   buildDailyScheduleSettingsUpdate,
   persistDailyScheduleSettingsUpdate
 } from "../src/renderer/App";
@@ -21,6 +22,22 @@ describe("daily schedule settings form", () => {
     });
   });
 
+  it.each(["", "8:15", "24:00", "12:60"])("reports the invalid time %j when activation is invoked directly", async (time) => {
+    const persist = vi.fn();
+    const showError = vi.fn();
+
+    await expect(activateDailyScheduleSettings(
+      time,
+      "today",
+      persist,
+      showError,
+      new Date(2026, 7, 22, 18, 30, 0, 0)
+    )).resolves.toBe(false);
+
+    expect(showError).toHaveBeenCalledWith("Bitte eine gültige Startzeit auswählen.");
+    expect(persist).not.toHaveBeenCalled();
+  });
+
   it("applies persisted settings after a successful activation", async () => {
     const persisted = { dailyStartEnabled: true } as RendererSettings;
     const updateSettings = vi.fn().mockResolvedValue(persisted);