diff --git a/apps/web/src/components/dashboard/widgets/calendar-month.test.ts b/apps/web/src/components/dashboard/widgets/calendar-month.test.ts
new file mode 100644
index 0000000..49bc607
--- /dev/null
+++ b/apps/web/src/components/dashboard/widgets/calendar-month.test.ts
@@ -0,0 +1,142 @@
+import { describe, expect, it } from 'vitest';
+import type { CalendarEvent } from '@/lib/calendar-api';
+import {
+  addDays,
+  buildCalendarDays,
+  computeFetchWindow,
+  dateKey,
+  formatEventDate,
+  formatEventTime,
+  formatMonthLabel,
+  gridStartFor,
+  groupEventsByDate,
+  resolveCalendarConfig,
+  selectUpcomingEvents,
+  startOfLocalDay,
+} from './calendar-month';
+
+/** Kleine Fabrik fuer Testtermine (Muster aus dem Plan). */
+function ev(id: string, start: Date, end: Date, extra?: Partial<CalendarEvent>): CalendarEvent {
+  return {
+    id,
+    sourceId: 's1',
+    title: id,
+    start: start.toISOString(),
+    end: end.toISOString(),
+    allDay: false,
+    ...extra,
+  };
+}
+
+describe('calendar-month', () => {
+  it('Test 1: buildCalendarDays liefert 42 Zellen ab Montag mit korrektem Monatsflag und heutigem Tag', () => {
+    const days = buildCalendarDays(new Date(2026, 6, 1), new Map(), new Date(2026, 6, 15));
+
+    expect(days).toHaveLength(42);
+    expect(days[0].key).toBe('2026-06-29');
+    expect(days[0].inCurrentMonth).toBe(false);
+    expect(days[2].key).toBe('2026-07-01');
+    expect(days[2].inCurrentMonth).toBe(true);
+    expect(days[41].key).toBe('2026-08-09');
+
+    const todayCells = days.filter((d) => d.isToday);
+    expect(todayCells).toHaveLength(1);
+    expect(todayCells[0].key).toBe('2026-07-15');
+
+    const outside = days.filter((d) => !d.inCurrentMonth);
+    expect(outside).toHaveLength(11);
+  });
+
+  it('Test 2: groupEventsByDate gruppiert nach lokalem Startdatum, buildCalendarDays uebernimmt die Zaehlung', () => {
+    const events = [
+      ev('e1', new Date(2026, 6, 20, 9, 0), new Date(2026, 6, 20, 10, 0)),
+      ev('e2', new Date(2026, 6, 20, 14, 0), new Date(2026, 6, 20, 15, 0)),
+      ev('e3', new Date(2026, 6, 21, 10, 0), new Date(2026, 6, 21, 11, 0)),
+    ];
+
+    const grouped = groupEventsByDate(events);
+    expect(grouped.size).toBe(2);
+    expect(grouped.get('2026-07-20')).toHaveLength(2);
+
+    const days = buildCalendarDays(new Date(2026, 6, 1), grouped);
+    const day20 = days.find((d) => d.key === '2026-07-20');
+    expect(day20?.events.length).toBe(2);
+  });
+
+  it('Test 3: resolveCalendarConfig klemmt ungueltige Werte auf die Vorgaben/Grenzen zurueck', () => {
+    expect(resolveCalendarConfig({})).toEqual({ showMonth: true, maxEvents: 3, lookaheadDays: 30 });
+    expect(resolveCalendarConfig({ showMonth: false }).showMonth).toBe(false);
+    expect(resolveCalendarConfig({ showMonth: 'nein' }).showMonth).toBe(true);
+    expect(resolveCalendarConfig({ maxEvents: 99 }).maxEvents).toBe(10);
+    expect(resolveCalendarConfig({ maxEvents: -1 }).maxEvents).toBe(0);
+    expect(resolveCalendarConfig({ maxEvents: 4.7 }).maxEvents).toBe(4);
+    expect(resolveCalendarConfig({ maxEvents: '5' }).maxEvents).toBe(3);
+    expect(resolveCalendarConfig({ lookaheadDays: 45 }).lookaheadDays).toBe(30);
+    expect(resolveCalendarConfig({ lookaheadDays: 90 }).lookaheadDays).toBe(90);
+  });
+
+  it('Test 4: computeFetchWindow deckt Raster und Vorschau-Zeitraum als Tagesgrenzen ab', () => {
+    const w1 = computeFetchWindow(new Date(2026, 6, 1), 30, new Date(2026, 6, 15, 10, 30));
+    expect(w1.from.getTime()).toBe(new Date(2026, 5, 29).getTime());
+    expect(w1.to.getTime()).toBe(new Date(2026, 7, 14).getTime());
+
+    const w2 = computeFetchWindow(new Date(2026, 6, 1), 30, new Date(2026, 4, 1, 8, 0));
+    expect(w2.from.getTime()).toBe(new Date(2026, 4, 1).getTime());
+    expect(w2.to.getTime()).toBe(new Date(2026, 7, 10).getTime());
+
+    const w3 = computeFetchWindow(new Date(2026, 6, 1), 90, new Date(2026, 6, 15, 10, 30));
+    expect(w3.to.getTime()).toBe(new Date(2026, 9, 13).getTime());
+
+    for (const w of [w1, w2, w3]) {
+      for (const d of [w.from, w.to]) {
+        expect(d.getHours()).toBe(0);
+        expect(d.getMinutes()).toBe(0);
+        expect(d.getSeconds()).toBe(0);
+        expect(d.getMilliseconds()).toBe(0);
+      }
+    }
+  });
+
+  it('Test 5: selectUpcomingEvents filtert Vergangenheit/Horizont, sortiert nach Start und kuerzt auf maxEvents', () => {
+    const now = new Date(2026, 6, 15, 10, 0);
+    const events = [
+      ev('gestern', new Date(2026, 6, 14, 11, 0), new Date(2026, 6, 14, 12, 0)),
+      ev('läuft', new Date(2026, 6, 15, 9, 0), new Date(2026, 6, 15, 11, 0)),
+      ev('heute 15', new Date(2026, 6, 15, 15, 0), new Date(2026, 6, 15, 16, 0)),
+      ev('in 5 Tagen', new Date(2026, 6, 20, 9, 0), new Date(2026, 6, 20, 10, 0)),
+      ev('in 10 Tagen', new Date(2026, 6, 25, 9, 0), new Date(2026, 6, 25, 10, 0)),
+    ];
+
+    const result = selectUpcomingEvents(events, 7, 2, now);
+    expect(result.map((e) => e.title)).toEqual(['läuft', 'heute 15']);
+
+    expect(selectUpcomingEvents(events, 7, 0, now)).toEqual([]);
+  });
+
+  it('Test 6: formatEventDate/formatEventTime/formatMonthLabel formatieren de-DE', () => {
+    const timed = ev('t1', new Date(2026, 6, 20, 9, 5), new Date(2026, 6, 20, 10, 0));
+    const timedText = formatEventDate(timed);
+    expect(timedText).toMatch(/20\.07\./);
+    expect(timedText).toMatch(/09:05/);
+
+    const allDay = ev('t2', new Date(2026, 6, 20, 0, 0), new Date(2026, 6, 20, 23, 59), { allDay: true });
+    const allDayText = formatEventDate(allDay);
+    expect(allDayText).toMatch(/20\.07\./);
+    expect(allDayText).not.toMatch(/\d{2}:\d{2}/);
+
+    expect(formatMonthLabel(new Date(2026, 6, 1))).toBe('Juli 2026');
+    expect(formatEventTime(new Date(2026, 6, 20, 9, 5).toISOString())).toBe('09:05');
+  });
+
+  it('Test 7: gridStartFor findet den Montag am oder vor dem 1. des Monats', () => {
+    expect(gridStartFor(new Date(2026, 7, 1)).getTime()).toBe(new Date(2026, 6, 27).getTime());
+    expect(gridStartFor(new Date(2026, 5, 1)).getTime()).toBe(new Date(2026, 5, 1).getTime());
+  });
+
+  it('Zusatz: dateKey/startOfLocalDay/addDays arbeiten lokal ohne UTC-Versatz', () => {
+    expect(dateKey(new Date(2026, 6, 5))).toBe('2026-07-05');
+    const start = startOfLocalDay(new Date(2026, 6, 5, 14, 30, 15));
+    expect(start.getHours()).toBe(0);
+    expect(addDays(new Date(2026, 6, 30), 2).getDate()).toBe(1);
+  });
+});
diff --git a/apps/web/src/components/dashboard/widgets/calendar-month.ts b/apps/web/src/components/dashboard/widgets/calendar-month.ts
new file mode 100644
index 0000000..ceb887e
--- /dev/null
+++ b/apps/web/src/components/dashboard/widgets/calendar-month.ts
@@ -0,0 +1,222 @@
+/**
+ * calendar-month — reine Hilfsfunktionen fuer das Kalender-Widget
+ * (quick-260916-htc).
+ *
+ * Kein React, keine DOM-Zugriffe: dieses Modul wird sowohl vom Widget
+ * (calendar-widget.tsx) als auch vom Einstellungsfeld
+ * (widget-settings-panel.tsx, `CalendarConfig`) importiert, damit beide
+ * Stellen dieselben Vorgaben/Grenzen anwenden (Muster clock-font-size.ts,
+ * T-BWO-01 uebertragen auf T-HTC-01).
+ *
+ * Vorlage fuer Monatsraster/Terminliste: das alte persoenliche Dashboard
+ * des Anwenders (user-files/personal-dashboard/src/app/page.tsx).
+ *
+ * Starttag-Regel: mehrtaegige und ganztaegige Termine werden im Monatsraster
+ * bewusst NUR am Starttag gezaehlt/angezeigt (`groupEventsByDate` gruppiert
+ * ausschliesslich nach `event.start`) — eine Terminleiste ueber mehrere Tage
+ * ist nicht Teil dieses Auftrags.
+ */
+
+import type { CalendarEvent } from '@/lib/calendar-api';
+
+export const CALENDAR_LOOKAHEAD_OPTIONS: readonly number[] = [7, 14, 30, 60, 90];
+export const CALENDAR_MAX_EVENTS_LIMIT = 10;
+export const CALENDAR_DEFAULTS = { showMonth: true, maxEvents: 3, lookaheadDays: 30 } as const;
+export const WEEKDAY_LABELS = ['Mo', 'Di', 'Mi', 'Do', 'Fr', 'Sa', 'So'] as const;
+
+export interface CalendarWidgetConfig {
+  showMonth: boolean;
+  maxEvents: number;
+  lookaheadDays: number;
+}
+
+/**
+ * Loest die Kalender-Widget-Konfiguration aus dem rohen `config`-JSON auf und
+ * klemmt ungueltige Werte auf die Vorgaben zurueck (T-HTC-01). API prueft
+ * Config-Felder nicht (`@IsObject()`), deshalb entscheidet ALLEIN diese
+ * Funktion, was ein gueltiger Wert ist.
+ */
+export function resolveCalendarConfig(config: Record<string, unknown>): CalendarWidgetConfig {
+  const showMonth = config.showMonth !== false;
+
+  let maxEvents: number = CALENDAR_DEFAULTS.maxEvents;
+  if (typeof config.maxEvents === 'number' && Number.isFinite(config.maxEvents)) {
+    maxEvents = Math.min(CALENDAR_MAX_EVENTS_LIMIT, Math.max(0, Math.trunc(config.maxEvents)));
+  }
+
+  let lookaheadDays: number = CALENDAR_DEFAULTS.lookaheadDays;
+  if (
+    typeof config.lookaheadDays === 'number' &&
+    CALENDAR_LOOKAHEAD_OPTIONS.includes(config.lookaheadDays)
+  ) {
+    lookaheadDays = config.lookaheadDays;
+  }
+
+  return { showMonth, maxEvents, lookaheadDays };
+}
+
+/** Lokaler Tagesschluessel `YYYY-MM-DD` (kein UTC-Versatz). */
+export function dateKey(date: Date): string {
+  const year = date.getFullYear();
+  const month = String(date.getMonth() + 1).padStart(2, '0');
+  const day = String(date.getDate()).padStart(2, '0');
+  return `${year}-${month}-${day}`;
+}
+
+/** Kopie von `date` auf die lokale Tagesgrenze 00:00:00.000 gesetzt. */
+export function startOfLocalDay(date: Date): Date {
+  const copy = new Date(date);
+  copy.setHours(0, 0, 0, 0);
+  return copy;
+}
+
+/**
+ * Kopie von `date`, um `days` Tage verschoben. Rechnet ueber `setDate`, NICHT
+ * ueber Millisekunden — das haelt die lokale Wanduhrzeit ueber
+ * Sommerzeitwechsel hinweg stabil.
+ */
+export function addDays(date: Date, days: number): Date {
+  const copy = new Date(date);
+  copy.setDate(copy.getDate() + days);
+  return copy;
+}
+
+/** Montag am oder vor dem 1. des Monats von `monthDate`, 00:00 lokal. */
+export function gridStartFor(monthDate: Date): Date {
+  const firstDayOfMonth = new Date(monthDate.getFullYear(), monthDate.getMonth(), 1);
+  const mondayBasedStartOffset = (firstDayOfMonth.getDay() + 6) % 7;
+  const gridStart = new Date(firstDayOfMonth);
+  gridStart.setDate(firstDayOfMonth.getDate() - mondayBasedStartOffset);
+  gridStart.setHours(0, 0, 0, 0);
+  return gridStart;
+}
+
+export interface CalendarDay {
+  key: string;
+  date: Date;
+  inCurrentMonth: boolean;
+  isToday: boolean;
+  events: CalendarEvent[];
+}
+
+/**
+ * Gruppiert Termine nach lokalem Startdatum. Mehrtaegige/ganztaegige Termine
+ * zaehlen bewusst nur am Starttag (siehe Kopfkommentar).
+ */
+export function groupEventsByDate(events: CalendarEvent[]): Map<string, CalendarEvent[]> {
+  const groupedEvents = new Map<string, CalendarEvent[]>();
+  for (const event of events) {
+    const key = dateKey(new Date(event.start));
+    const existingEvents = groupedEvents.get(key) ?? [];
+    groupedEvents.set(key, [...existingEvents, event]);
+  }
+  return groupedEvents;
+}
+
+/** 42 Rasterzellen ab `gridStartFor(monthDate)`, Montag-basiert. */
+export function buildCalendarDays(
+  monthDate: Date,
+  eventsByDate: Map<string, CalendarEvent[]>,
+  today: Date = new Date(),
+): CalendarDay[] {
+  const month = monthDate.getMonth();
+  const gridStart = gridStartFor(monthDate);
+  const todayKey = dateKey(today);
+
+  const days: CalendarDay[] = [];
+  for (let index = 0; index < 42; index += 1) {
+    const date = addDays(gridStart, index);
+    const key = dateKey(date);
+    days.push({
+      key,
+      date,
+      inCurrentMonth: date.getMonth() === month,
+      isToday: key === todayKey,
+      events: eventsByDate.get(key) ?? [],
+    });
+  }
+  return days;
+}
+
+/**
+ * Ladefenster fuer `fetchEvents(from, to)`: deckt sowohl das 42-Tage-Raster
+ * als auch den Vorschau-Zeitraum (lookaheadDays ab heute) ab. Alle vier
+ * Zwischenwerte sind Tagesgrenzen 00:00 lokal, deshalb ist `toISOString()`
+ * innerhalb eines Tages konstant — das haelt den Backend-Cache-Schluessel
+ * (`${userId}:${from.toISOString()}:${to.toISOString()}`) ueber den
+ * 5-Minuten-Refresh hinweg stabil.
+ */
+export function computeFetchWindow(
+  monthDate: Date,
+  lookaheadDays: number,
+  now: Date = new Date(),
+): { from: Date; to: Date } {
+  const gridStart = gridStartFor(monthDate);
+  const gridEnd = addDays(gridStart, 42);
+  const todayStart = startOfLocalDay(now);
+  const lookEnd = addDays(todayStart, lookaheadDays);
+
+  const from = gridStart.getTime() <= todayStart.getTime() ? gridStart : todayStart;
+  const to = gridEnd.getTime() >= lookEnd.getTime() ? gridEnd : lookEnd;
+
+  return { from, to };
+}
+
+/**
+ * Waehlt die naechsten anstehenden Termine (laufend oder in der Zukunft,
+ * innerhalb `lookaheadDays`), sortiert nach Start aufsteigend, gekuerzt auf
+ * `maxEvents`.
+ */
+export function selectUpcomingEvents(
+  events: CalendarEvent[],
+  lookaheadDays: number,
+  maxEvents: number,
+  now: Date = new Date(),
+): CalendarEvent[] {
+  const horizon = addDays(startOfLocalDay(now), lookaheadDays).getTime();
+  const nowTime = now.getTime();
+
+  return events
+    .filter((event) => {
+      const endTime = new Date(event.end).getTime();
+      const startTime = new Date(event.start).getTime();
+      return endTime >= nowTime && startTime < horizon;
+    })
+    .sort((a, b) => new Date(a.start).getTime() - new Date(b.start).getTime())
+    .slice(0, maxEvents);
+}
+
+/** Datum+Uhrzeit (bzw. nur Datum bei ganztaegig) im Format „Mi., 20.07., 09:05“. */
+export function formatEventDate(event: CalendarEvent): string {
+  const start = new Date(event.start);
+  if (event.allDay) {
+    return new Intl.DateTimeFormat('de-DE', {
+      weekday: 'short',
+      day: '2-digit',
+      month: '2-digit',
+    }).format(start);
+  }
+  return new Intl.DateTimeFormat('de-DE', {
+    weekday: 'short',
+    day: '2-digit',
+    month: '2-digit',
+    hour: '2-digit',
+    minute: '2-digit',
+  }).format(start);
+}
+
+/** Nur die Uhrzeit eines ISO-Zeitstempels, „09:05“. */
+export function formatEventTime(iso: string): string {
+  return new Intl.DateTimeFormat('de-DE', {
+    hour: '2-digit',
+    minute: '2-digit',
+  }).format(new Date(iso));
+}
+
+/** Monat+Jahr, „Juli 2026“. */
+export function formatMonthLabel(date: Date): string {
+  return new Intl.DateTimeFormat('de-DE', {
+    month: 'long',
+    year: 'numeric',
+  }).format(date);
+}
diff --git a/apps/web/src/messages/de.json b/apps/web/src/messages/de.json
index e53cf4f..329c343 100644
--- a/apps/web/src/messages/de.json
+++ b/apps/web/src/messages/de.json
@@ -208,6 +208,22 @@
       "loading": "Laden...",
       "emptyNoSources": "Keine Kalenderquellen konfiguriert",
       "emptyNoEvents": "Keine anstehenden Termine",
+      "nothingSelected": "Nichts zum Anzeigen ausgewählt",
+      "monthPrev": "Zurück",
+      "monthNext": "Weiter",
+      "monthToday": "Zurück zum heutigen Monat",
+      "upcomingTitle": "Nächste Termine",
+      "tooltipMore": "Weitere Termine vorhanden",
+      "allDay": "ganztägig",
+      "configShowMonth": "Monatsansicht anzeigen",
+      "configMaxEvents": "Anzahl Termine",
+      "configMaxEventsNone": "Ausblenden",
+      "configMaxEventsOne": "1 Termin",
+      "configMaxEventsMany": "{count} Termine",
+      "configLookahead": "Zeitraum",
+      "configLookaheadOption": "Nächste {days} Tage",
+      "configSourcesHint": "Kalenderquellen verwalten Sie unter",
+      "configSourcesLink": "Einstellungen → Dashboard → Kalender",
       "connectionSuccess": "Verbindung erfolgreich",
       "connectionError": "Verbindung fehlgeschlagen. Bitte überprüfen Sie die URL und Zugangsdaten.",
       "addSource": "Quelle hinzufügen",
diff --git a/apps/web/src/messages/en.json b/apps/web/src/messages/en.json
index b4ab0e9..ccc6bed 100644
--- a/apps/web/src/messages/en.json
+++ b/apps/web/src/messages/en.json
@@ -208,6 +208,22 @@
       "loading": "Loading...",
       "emptyNoSources": "No calendar sources configured",
       "emptyNoEvents": "No upcoming events",
+      "nothingSelected": "Nothing selected to display",
+      "monthPrev": "Back",
+      "monthNext": "Next",
+      "monthToday": "Back to the current month",
+      "upcomingTitle": "Upcoming events",
+      "tooltipMore": "More events available",
+      "allDay": "all day",
+      "configShowMonth": "Show month view",
+      "configMaxEvents": "Number of events",
+      "configMaxEventsNone": "Hide",
+      "configMaxEventsOne": "1 event",
+      "configMaxEventsMany": "{count} events",
+      "configLookahead": "Time range",
+      "configLookaheadOption": "Next {days} days",
+      "configSourcesHint": "Manage calendar sources under",
+      "configSourcesLink": "Settings → Dashboard → Calendar",
       "connectionSuccess": "Connection successful",
       "connectionError": "Connection failed. Please check the URL and credentials.",
       "addSource": "Add source",