apps/bot/src/modules/scheduler/service.ts

@@ -4,11 +4,20 @@ import {
   type MessageCreateOptions,
   type TextChannel
 } from 'discord.js';
-import { WelcomeEmbedSchema, expandBracketChannelMentions, parseMessageComponentsV2, t, tf } from '@nexumi/shared';
+import {
+  DEFAULT_GUILD_TIMEZONE,
+  WelcomeEmbedSchema,
+  expandBracketChannelMentions,
+  parseMessageComponentsV2,
+  resolveScheduleRunAt,
+  t,
+  tf
+} from '@nexumi/shared';
 import type { ScheduledMessage } from '@prisma/client';
 import { ensureGuild } from '../../guild.js';
 import { buildEmbedFromPayload } from '../../lib/embed-payload.js';
 import { buildComponentsV2Payload } from '../../lib/components-v2-payload.js';
+import { safeRemoveBullJob } from '../../lib/safe-remove-job.js';
 import { logger } from '../../logger.js';
 import { scheduleQueue } from '../../queues.js';
 import type { BotContext } from '../../types.js';
@@ -36,7 +45,7 @@ export type ScheduleCreateInput = AnnouncementInput & {
   cron?: string | null;
 };
 
-function parseScheduleWhen(input: string): Date {
+function parseScheduleWhen(input: string, timeZone: string): Date {
   const trimmed = input.trim();
   if (/^\d+[smhd]$/i.test(trimmed)) {
     const ms = parseDuration(trimmed);
@@ -46,11 +55,27 @@ function parseScheduleWhen(input: string): Date {
     return new Date(Date.now() + ms);
   }
 
-  const parsed = Date.parse(trimmed);
+  try {
-  if (Number.isNaN(parsed) || parsed <= Date.now()) {
+    const parsed = resolveScheduleRunAt(trimmed, timeZone);
+    if (parsed.getTime() <= Date.now()) {
+      throw new SchedulerError('invalid_when');
+    }
+    return parsed;
+  } catch {
     throw new SchedulerError('invalid_when');
   }
-  return new Date(parsed);
+}
+
+async function getGuildTimezone(prisma: BotContext['prisma'], guildId: string): Promise<string> {
+  const settings = await prisma.guildSettings.findUnique({
+    where: { guildId },
+    select: { timezone: true }
+  });
+  return settings?.timezone || DEFAULT_GUILD_TIMEZONE;
+}
+
+export function scheduleJobId(scheduleId: string): string {
+  return `schedule-${scheduleId}`;
 }
 
 function isValidCron(pattern: string): boolean {
@@ -157,53 +182,74 @@ async function assertSendableChannel(
 async function enqueueScheduleJob(
   schedule: ScheduledMessage,
   cron: string | null,
-  runAt: Date | null
+  runAt: Date | null,
+  timeZone: string
 ): Promise<string> {
-  const jobOptions: {
+  const jobId = scheduleJobId(schedule.id);
-    jobId: string;
-    removeOnComplete: boolean;
-    removeOnFail: number;
-    delay?: number;
-    repeat?: { pattern: string };
-  } = {
-    jobId: `schedule-${schedule.id}`,
-    removeOnComplete: true,
-    removeOnFail: 50
-  };
 
   if (cron) {
-    jobOptions.repeat = { pattern: cron };
+    // BullMQ 5 job schedulers honor guild timezone for cron patterns.
-  } else if (runAt) {
+    await scheduleQueue.upsertJobScheduler(
-    jobOptions.delay = Math.max(0, runAt.getTime() - Date.now());
+      jobId,
+      { pattern: cron, tz: timeZone },
+      {
+        name: 'scheduleSend',
+        data: { scheduleId: schedule.id },
+        opts: {
+          removeOnComplete: true,
+          removeOnFail: 50
+        }
+      }
+    );
+    return jobId;
   }
 
-  const job = await scheduleQueue.add('scheduleSend', { scheduleId: schedule.id }, jobOptions);
+  if (!runAt) {
+    throw new SchedulerError('missing_schedule_time');
+  }
+
+  const job = await scheduleQueue.add(
+    'scheduleSend',
+    { scheduleId: schedule.id },
+    {
+      jobId,
+      removeOnComplete: true,
+      removeOnFail: 50,
+      delay: Math.max(0, runAt.getTime() - Date.now())
+    }
+  );
   return String(job.id);
 }
 
 /**
- * Removes a scheduled/failed job if present. Locked (active) jobs cannot be
+ * Removes a one-shot delayed job and/or cron job scheduler. Locked (active)
- * removed — do not call this from inside the scheduleSend worker for the
+ * jobs cannot be removed — do not call this from inside the scheduleSend
- * job that is currently running.
+ * worker for the job that is currently running.
  */
-export async function removeScheduleJob(jobId: string | null): Promise<void> {
+export async function removeScheduleJob(
-  if (!jobId) {
+  jobId: string | null,
-    return;
+  options?: { scheduleId?: string; cron?: string | null }
+): Promise<void> {
+  const schedulerId = options?.scheduleId ? scheduleJobId(options.scheduleId) : jobId;
+  if (schedulerId && options?.cron) {
+    try {
+      await scheduleQueue.removeJobScheduler(schedulerId);
+    } catch (error) {
+      logger.warn({ schedulerId, error }, 'Failed to remove schedule job scheduler');
+    }
   }
 
-  const job = await scheduleQueue.getJob(jobId);
+  const candidateIds = new Set<string>();
-  if (!job) {
+  if (jobId) {
-    return;
+    candidateIds.add(jobId);
   }
-  try {
+  if (schedulerId) {
-    await job.remove();
+    candidateIds.add(schedulerId);
-  } catch (error) {
+  }
-    const message = error instanceof Error ? error.message : String(error);
+
-    if (message.includes('locked') || message.includes('could not be removed')) {
+  for (const id of candidateIds) {
-      logger.warn({ jobId, error }, 'Schedule job is locked; skipping remove');
+    const job = await scheduleQueue.getJob(id);
-      return;
+    await safeRemoveBullJob(job, { label: 'schedule', jobId: id });
-    }
-    throw error;
   }
 }
 
@@ -235,6 +281,7 @@ export async function createScheduledMessage(
 
   let runAt: Date | null = null;
   let cronPattern: string | null = null;
+  const timeZone = await getGuildTimezone(context.prisma, input.guildId);
 
   if (cron) {
     if (!isValidCron(cron)) {
@@ -242,7 +289,7 @@ export async function createScheduledMessage(
     }
     cronPattern = cron;
   } else if (whenInput) {
-    runAt = parseScheduleWhen(whenInput);
+    runAt = parseScheduleWhen(whenInput, timeZone);
   }
 
   await assertSendableChannel(context, input.channelId);
@@ -262,7 +309,7 @@ export async function createScheduledMessage(
     }
   });
 
-  const jobId = await enqueueScheduleJob(schedule, cronPattern, runAt);
+  const jobId = await enqueueScheduleJob(schedule, cronPattern, runAt, timeZone);
   return context.prisma.scheduledMessage.update({
     where: { id: schedule.id },
     data: { jobId }
@@ -322,7 +369,7 @@ export async function deleteScheduledMessage(
     return false;
   }
 
-  await removeScheduleJob(match.jobId);
+  await removeScheduleJob(match.jobId, { scheduleId: match.id, cron: match.cron });
   await context.prisma.scheduledMessage.delete({ where: { id: match.id } });
   return true;
 }

apps/webui/src/app/globals.css

@@ -3,6 +3,7 @@
 @custom-variant dark (&:is(.dark *));
 
 :root {
+  color-scheme: light;
   --background: #f8f8fb;
   --foreground: #111116;
   --card: #ffffff;
@@ -26,6 +27,7 @@
 }
 
 .dark {
+  color-scheme: dark;
   --background: #0a0a0d;
   --foreground: #e6e6ea;
   --card: #131318;

apps/webui/src/components/modules/scheduler-manager.tsx

@@ -29,10 +29,12 @@ import { Textarea } from '@/components/ui/textarea';
 import { cn } from '@/lib/utils';
 
 type ScheduleContentMode = 'text' | 'embed' | 'components_v2';
+type ScheduleTimingMode = 'once' | 'cron';
 
 const EMPTY_DRAFT = {
   channelId: '',
   contentMode: 'text' as ScheduleContentMode,
+  timingMode: 'once' as ScheduleTimingMode,
   content: '',
   embed: null as WelcomeEmbed | null,
   components: null as MessageComponentsV2 | null,
@@ -76,6 +78,19 @@ function schedulePreview(schedule: ScheduledMessageDashboard, t: (key: string) =
   return t('modulePages.scheduler.noContent');
 }
 
+function scheduleErrorMessage(code: string | undefined, t: (key: string) => string): string {
+  if (code === 'invalid_cron') {
+    return t('modulePages.scheduler.errors.invalidCron');
+  }
+  if (code === 'invalid_when') {
+    return t('modulePages.scheduler.errors.invalidWhen');
+  }
+  if (code === 'missing_schedule_time') {
+    return t('modulePages.scheduler.errors.missingScheduleTime');
+  }
+  return code?.trim() ? code : t('common.saveError');
+}
+
 interface SchedulerManagerProps {
   guildId: string;
   initialSchedules: ScheduledMessageDashboard[];
@@ -98,7 +113,10 @@ export function SchedulerManager({ guildId, initialSchedules }: SchedulerManager
       (draft.contentMode === 'embed' && (embedHasContent(embed) || embed?.color !== undefined)) ||
       (draft.contentMode === 'components_v2' && componentsV2HasContent(components));
 
-    if (!draft.channelId.trim() || !hasContent || (!draft.cron.trim() && !draft.runAt)) {
+    const cron = draft.timingMode === 'cron' ? draft.cron.trim() : '';
+    const runAt = draft.timingMode === 'once' ? draft.runAt.trim() : '';
+
+    if (!draft.channelId.trim() || !hasContent || (!cron && !runAt)) {
       toast.error(t('modulePages.scheduler.formIncomplete'));
       return;
     }
@@ -114,15 +132,16 @@ export function SchedulerManager({ guildId, initialSchedules }: SchedulerManager
           embed,
           components,
           rolePingId: draft.rolePingId.trim() || undefined,
-          cron: draft.cron.trim() || null,
+          // Send naive datetime-local; server interprets it in the guild timezone.
-          runAt: draft.cron.trim() ? null : draft.runAt ? new Date(draft.runAt).toISOString() : null
+          cron: cron || null,
+          runAt: cron ? null : runAt || null
         })
       });
       const body = (await response.json().catch(() => null)) as
         | (ScheduledMessageDashboard & { error?: string })
         | null;
       if (!response.ok || !body) {
-        toast.error(body?.error ?? t('common.saveError'));
+        toast.error(scheduleErrorMessage(body?.error, t));
         return;
       }
       setSchedules((prev) => [...prev, body]);
@@ -179,24 +198,55 @@ export function SchedulerManager({ guildId, initialSchedules }: SchedulerManager
                 placeholder={t('common.optional')}
               />
             </div>
+          </div>
+
+          <div className="space-y-2">
+            <Label>{t('modulePages.scheduler.timingMode')}</Label>
+            <div className="flex flex-wrap gap-2">
+              {(['once', 'cron'] as ScheduleTimingMode[]).map((mode) => (
+                <Button
+                  key={mode}
+                  type="button"
+                  variant={draft.timingMode === mode ? 'default' : 'outline'}
+                  size="sm"
+                  className={cn(draft.timingMode !== mode && 'text-muted-foreground')}
+                  onClick={() =>
+                    setDraft((prev) => ({
+                      ...prev,
+                      timingMode: mode,
+                      cron: mode === 'cron' ? prev.cron : '',
+                      runAt: mode === 'once' ? prev.runAt : ''
+                    }))
+                  }
+                >
+                  {t(`modulePages.scheduler.timingModes.${mode}`)}
+                </Button>
+              ))}
+            </div>
+          </div>
+
+          {draft.timingMode === 'cron' ? (
             <div className="space-y-2">
-              <Label>{t('modulePages.scheduler.cron')}</Label>
+              <Label htmlFor="scheduler-cron">{t('modulePages.scheduler.cron')}</Label>
               <Input
+                id="scheduler-cron"
                 value={draft.cron}
                 onChange={(event) => setDraft((prev) => ({ ...prev, cron: event.target.value }))}
                 placeholder="0 9 * * *"
+                autoComplete="off"
               />
             </div>
+          ) : (
             <div className="space-y-2">
-              <Label>{t('modulePages.scheduler.runAt')}</Label>
+              <Label htmlFor="scheduler-run-at">{t('modulePages.scheduler.runAt')}</Label>
               <Input
+                id="scheduler-run-at"
                 type="datetime-local"
                 value={draft.runAt}
-                disabled={Boolean(draft.cron.trim())}
                 onChange={(event) => setDraft((prev) => ({ ...prev, runAt: event.target.value }))}
               />
             </div>
-          </div>
+          )}
 
           <div className="space-y-2">
             <Label>{t('modulePages.scheduler.contentMode')}</Label>

apps/webui/src/components/ui/input.tsx

@@ -9,7 +9,7 @@ const Input = React.forwardRef<HTMLInputElement, InputProps>(({ className, type,
       type={type}
       data-slot="input"
       className={cn(
-        'flex h-9 w-full rounded-md border border-input bg-transparent px-3 py-1 text-sm shadow-sm transition-colors placeholder:text-muted-foreground focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 focus-visible:ring-offset-background disabled:cursor-not-allowed disabled:opacity-50',
+        'flex h-9 w-full rounded-md border border-input bg-transparent px-3 py-1 text-sm text-foreground shadow-sm transition-colors placeholder:text-muted-foreground focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 focus-visible:ring-offset-background disabled:cursor-not-allowed disabled:opacity-50',
         className
       )}
       ref={ref}

apps/webui/src/lib/module-configs/scheduler.ts

@@ -1,6 +1,8 @@
 import {
+  DEFAULT_GUILD_TIMEZONE,
   MessageComponentsV2Schema,
   WelcomeEmbedSchema,
+  resolveScheduleRunAt,
   type MessageComponentsV2,
   type ScheduledMessageDashboard,
   type ScheduledMessageDashboardCreate,
@@ -42,6 +44,10 @@ function toDashboard(schedule: ScheduledMessage): ScheduledMessageDashboard {
   };
 }
 
+function scheduleJobId(scheduleId: string): string {
+  return `schedule-${scheduleId}`;
+}
+
 export async function listScheduledMessagesDashboard(guildId: string): Promise<ScheduledMessageDashboard[]> {
   const schedules = await prisma.scheduledMessage.findMany({
     where: { guildId, enabled: true },
@@ -56,6 +62,14 @@ function isValidCron(pattern: string): boolean {
   return parts.length >= 5 && parts.length <= 6;
 }
 
+async function getGuildTimezone(guildId: string): Promise<string> {
+  const settings = await prisma.guildSettings.findUnique({
+    where: { guildId },
+    select: { timezone: true }
+  });
+  return settings?.timezone || DEFAULT_GUILD_TIMEZONE;
+}
+
 /**
  * Mirrors `apps/bot/src/modules/scheduler/service.ts#createScheduledMessage`
  * job scheduling exactly (same queue, same `scheduleSend` job name, same
@@ -77,7 +91,15 @@ export async function createScheduledMessageDashboard(
     throw new SchedulerValidationError('invalid_cron');
   }
 
-  const runAt = !cron && input.runAt ? new Date(input.runAt) : null;
+  const timeZone = await getGuildTimezone(guildId);
+  let runAt: Date | null = null;
+  if (!cron && input.runAt) {
+    try {
+      runAt = resolveScheduleRunAt(input.runAt, timeZone);
+    } catch {
+      throw new SchedulerValidationError('invalid_when');
+    }
+  }
   if (!cron && !runAt) {
     throw new SchedulerValidationError('missing_schedule_time');
   }
@@ -102,27 +124,38 @@ export async function createScheduledMessageDashboard(
     }
   });
 
-  const jobOptions: {
+  const jobId = scheduleJobId(schedule.id);
-    jobId: string;
+  const queue = getScheduleQueue();
-    removeOnComplete: boolean;
+
-    removeOnFail: number;
-    delay?: number;
-    repeat?: { pattern: string };
-  } = {
-    jobId: `schedule-${schedule.id}`,
-    removeOnComplete: true,
-    removeOnFail: 50
-  };
   if (cron) {
-    jobOptions.repeat = { pattern: cron };
+    await queue.upsertJobScheduler(
+      jobId,
+      { pattern: cron, tz: timeZone },
+      {
+        name: 'scheduleSend',
+        data: { scheduleId: schedule.id },
+        opts: {
+          removeOnComplete: true,
+          removeOnFail: 50
+        }
+      }
+    );
   } else if (runAt) {
-    jobOptions.delay = Math.max(0, runAt.getTime() - Date.now());
+    await queue.add(
+      'scheduleSend',
+      { scheduleId: schedule.id },
+      {
+        jobId,
+        removeOnComplete: true,
+        removeOnFail: 50,
+        delay: Math.max(0, runAt.getTime() - Date.now())
+      }
+    );
   }
 
-  const job = await getScheduleQueue().add('scheduleSend', { scheduleId: schedule.id }, jobOptions);
   const updated = await prisma.scheduledMessage.update({
     where: { id: schedule.id },
-    data: { jobId: String(job.id) }
+    data: { jobId }
   });
 
   return toDashboard(updated);
@@ -134,16 +167,34 @@ export async function deleteScheduledMessageDashboard(guildId: string, scheduleI
     return false;
   }
 
+  const queue = getScheduleQueue();
+  const schedulerId = scheduleJobId(existing.id);
+
+  if (existing.cron) {
+    try {
+      await queue.removeJobScheduler(schedulerId);
+    } catch {
+      // Scheduler may already be gone (legacy repeatables / one-shots).
+    }
+  }
+
+  const candidateIds = new Set<string>();
+  candidateIds.add(schedulerId);
   if (existing.jobId) {
-    const job = await getScheduleQueue().getJob(existing.jobId);
+    candidateIds.add(existing.jobId);
-    if (job) {
+  }
-      try {
+
-        await job.remove();
+  for (const id of candidateIds) {
-      } catch (error) {
+    const job = await queue.getJob(id);
-        const message = error instanceof Error ? error.message : String(error);
+    if (!job) {
-        if (!message.includes('locked') && !message.includes('could not be removed')) {
+      continue;
-          throw error;
+    }
-        }
+    try {
+      await job.remove();
+    } catch (error) {
+      const message = error instanceof Error ? error.message : String(error);
+      if (!message.includes('locked') && !message.includes('could not be removed')) {
+        throw error;
       }
     }
   }

apps/webui/src/messages/de.json

@@ -933,6 +933,11 @@
       "rolePingId": "Zu erwähnende Rolle",
       "cron": "Cron-Ausdruck",
       "runAt": "Ausführen am",
+      "timingMode": "Zeitplan",
+      "timingModes": {
+        "once": "Einmalig",
+        "cron": "Wiederkehrend (Cron)"
+      },
       "content": "Nachrichtentext",
       "embed": "Embed (optional)",
       "components": "Components-V2-Layout",
@@ -943,10 +948,15 @@
         "components_v2": "Components V2"
       },
       "componentsPreview": "(Components V2)",
-      "cronHint": "Cron-Ausdruck für wiederkehrende Nachrichten ODER ein einmaliges Datum/Uhrzeit — nicht beides. Wähle Text, Embed oder Components V2 (gegenseitig ausgeschlossen).",
+      "cronHint": "Einmalige Zeit oder Cron-Wiederholung — bezogen auf die Server-Zeitzone unter Allgemeine Einstellungen. Wähle Text, Embed oder Components V2 (gegenseitig ausgeschlossen).",
       "create": "Nachricht planen",
       "created": "Nachricht geplant.",
       "formIncomplete": "Bitte Kanal, Nachrichtenformat mit Inhalt sowie Cron-Ausdruck oder Datum/Uhrzeit ausfüllen.",
+      "errors": {
+        "invalidCron": "Ungültiger Cron-Ausdruck (5–6 Felder, z. B. 0 9 * * *).",
+        "invalidWhen": "Ungültiges Datum/Uhrzeit.",
+        "missingScheduleTime": "Bitte Cron-Ausdruck oder Datum/Uhrzeit angeben."
+      },
       "confirmDelete": "Diesen Zeitplan wirklich löschen?",
       "listTitle": "Geplante Nachrichten",
       "listDescription": "Anstehende und wiederkehrende geplante Nachrichten.",

apps/webui/src/messages/en.json

@@ -933,6 +933,11 @@
       "rolePingId": "Role to ping",
       "cron": "Cron expression",
       "runAt": "Run at",
+      "timingMode": "Schedule",
+      "timingModes": {
+        "once": "One-time",
+        "cron": "Recurring (cron)"
+      },
       "content": "Message content",
       "embed": "Embed (optional)",
       "components": "Components V2 layout",
@@ -943,10 +948,15 @@
         "components_v2": "Components V2"
       },
       "componentsPreview": "(Components V2)",
-      "cronHint": "Set a cron expression for recurring messages, or a one-time date/time — not both. Choose text, embed, or Components V2 (mutually exclusive).",
+      "cronHint": "One-time time or cron recurrence — evaluated in the server timezone under General Settings. Choose text, embed, or Components V2 (mutually exclusive).",
       "create": "Schedule message",
       "created": "Message scheduled.",
       "formIncomplete": "Please fill channel, choose a message format with content, and either a cron expression or a date/time.",
+      "errors": {
+        "invalidCron": "Invalid cron expression (5–6 fields, e.g. 0 9 * * *).",
+        "invalidWhen": "Invalid date/time.",
+        "missingScheduleTime": "Provide a cron expression or a date/time."
+      },
       "confirmDelete": "Really delete this schedule?",
       "listTitle": "Scheduled messages",
       "listDescription": "Upcoming and recurring scheduled messages.",

docs/PHASE-TRACKING.md

@@ -598,3 +598,20 @@
 - [ ] Online-Kanal zeigt sinnvolle Zahl (nicht dauerhaft 0)
 - [ ] Bot ohne „Kanäle verwalten“ → Config gespeichert; Bot-Log warnt; nach Permission-Fix erneut speichern
 
+## Post-Phase – Scheduler Cron / Run-at Fix (Status: implementiert)
+
+### Abgeschlossen (Code)
+
+- Cron und einmaliges „Ausführen am“ nutzen die Guild-Zeitzone (`GuildSettings.timezone`)
+- Cron über BullMQ `upsertJobScheduler` inkl. `tz`; Delete entfernt Job-Scheduler korrekt
+- Dashboard: Timing-Mode (einmalig vs. Cron), naive `datetime-local` an Server, Fehlertexte i18n
+- Dark Mode: `color-scheme` + Input `text-foreground` für datetime-local Lesbarkeit
+- Shared Helper `resolveScheduleRunAt` / `guildLocalDateTimeToUtc`
+
+### Manuell testen
+
+- [ ] WebUI einmalig: Datum/Uhrzeit in Server-Zeitzone → Nachricht zur erwarteten lokalen Zeit
+- [ ] WebUI Cron `0 9 * * *` bei Europe/Berlin → ca. 09:00 Berlin (nicht UTC)
+- [ ] Cron-Plan löschen → keine weiteren Sends
+- [ ] Dark Mode: Cron-Feld und datetime-local lesbar/bedienbar
+

packages/shared/src/index.ts

@@ -13,6 +13,7 @@ export * from './owner.js';
 export * from './public.js';
 export * from './premium.js';
 export * from './components-v2.js';
+export * from './timezone.js';
 
 export const LocaleSchema = z.enum(['de', 'en']);
 export type Locale = z.infer<typeof LocaleSchema>;

packages/shared/src/timezone.test.ts

@@ -0,0 +1,35 @@
+import { describe, expect, it } from 'vitest';
+import {
+  guildLocalDateTimeToUtc,
+  isValidTimezone,
+  resolveScheduleRunAt
+} from './timezone.js';
+
+describe('timezone helpers', () => {
+  it('validates IANA timezones', () => {
+    expect(isValidTimezone('Europe/Berlin')).toBe(true);
+    expect(isValidTimezone('Not/AZone')).toBe(false);
+  });
+
+  it('converts Berlin winter wall time to UTC', () => {
+    // 2026-01-15 09:00 Europe/Berlin = 08:00 UTC (CET, UTC+1)
+    const utc = guildLocalDateTimeToUtc('2026-01-15T09:00', 'Europe/Berlin');
+    expect(utc.toISOString()).toBe('2026-01-15T08:00:00.000Z');
+  });
+
+  it('converts Berlin summer wall time to UTC', () => {
+    // 2026-07-15 09:00 Europe/Berlin = 07:00 UTC (CEST, UTC+2)
+    const utc = guildLocalDateTimeToUtc('2026-07-15T09:00', 'Europe/Berlin');
+    expect(utc.toISOString()).toBe('2026-07-15T07:00:00.000Z');
+  });
+
+  it('keeps absolute ISO strings unchanged', () => {
+    const utc = resolveScheduleRunAt('2026-07-15T07:00:00.000Z', 'Europe/Berlin');
+    expect(utc.toISOString()).toBe('2026-07-15T07:00:00.000Z');
+  });
+
+  it('interprets naive datetime-local in guild timezone', () => {
+    const utc = resolveScheduleRunAt('2026-07-15T09:00', 'Europe/Berlin');
+    expect(utc.toISOString()).toBe('2026-07-15T07:00:00.000Z');
+  });
+});

packages/shared/src/timezone.ts

@@ -0,0 +1,117 @@
+/** Default guild timezone (matches Prisma `GuildSettings.timezone`). */
+export const DEFAULT_GUILD_TIMEZONE = 'Europe/Berlin';
+
+export function isValidTimezone(timezone: string): boolean {
+  try {
+    Intl.DateTimeFormat(undefined, { timeZone: timezone });
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+type ZonedParts = {
+  year: number;
+  month: number;
+  day: number;
+  hour: number;
+  minute: number;
+  second: number;
+};
+
+function getZonedParts(date: Date, timeZone: string): ZonedParts {
+  const formatter = new Intl.DateTimeFormat('en-US', {
+    timeZone,
+    year: 'numeric',
+    month: '2-digit',
+    day: '2-digit',
+    hour: '2-digit',
+    minute: '2-digit',
+    second: '2-digit',
+    hourCycle: 'h23'
+  });
+
+  const map: Partial<Record<Intl.DateTimeFormatPartTypes, string>> = {};
+  for (const part of formatter.formatToParts(date)) {
+    if (part.type !== 'literal') {
+      map[part.type] = part.value;
+    }
+  }
+
+  return {
+    year: Number(map.year),
+    month: Number(map.month),
+    day: Number(map.day),
+    hour: Number(map.hour),
+    minute: Number(map.minute),
+    second: Number(map.second)
+  };
+}
+
+const NAIVE_LOCAL_RE = /^(\d{4})-(\d{2})-(\d{2})[T ](\d{2}):(\d{2})(?::(\d{2}))?$/;
+
+/**
+ * Interprets a wall-clock datetime (`YYYY-MM-DDTHH:mm` / with seconds / space
+ * separator) as being in `timeZone` and returns the corresponding UTC `Date`.
+ */
+export function guildLocalDateTimeToUtc(localDateTime: string, timeZone: string): Date {
+  const match = localDateTime.trim().match(NAIVE_LOCAL_RE);
+  if (!match) {
+    throw new RangeError('invalid_local_datetime');
+  }
+  if (!isValidTimezone(timeZone)) {
+    throw new RangeError('invalid_timezone');
+  }
+
+  const year = Number(match[1]);
+  const month = Number(match[2]);
+  const day = Number(match[3]);
+  const hour = Number(match[4]);
+  const minute = Number(match[5]);
+  const second = Number(match[6] ?? 0);
+
+  // Initial guess: treat the components as UTC, then correct by the TZ offset.
+  let utcMs = Date.UTC(year, month - 1, day, hour, minute, second);
+  for (let i = 0; i < 3; i += 1) {
+    const parts = getZonedParts(new Date(utcMs), timeZone);
+    const asUtc = Date.UTC(parts.year, parts.month - 1, parts.day, parts.hour, parts.minute, parts.second);
+    const targetAsUtc = Date.UTC(year, month - 1, day, hour, minute, second);
+    const diff = targetAsUtc - asUtc;
+    if (diff === 0) {
+      break;
+    }
+    utcMs += diff;
+  }
+
+  return new Date(utcMs);
+}
+
+/**
+ * Resolves a schedule `runAt` input to a UTC instant.
+ * - Values with `Z` / numeric offset are treated as absolute.
+ * - Naive `datetime-local` values are interpreted in the guild timezone.
+ */
+export function resolveScheduleRunAt(input: string, timeZone: string): Date {
+  const trimmed = input.trim();
+  if (!trimmed) {
+    throw new RangeError('invalid_when');
+  }
+
+  if (/([zZ]|[+-]\d{2}:?\d{2})$/.test(trimmed)) {
+    const parsed = new Date(trimmed);
+    if (Number.isNaN(parsed.getTime())) {
+      throw new RangeError('invalid_when');
+    }
+    return parsed;
+  }
+
+  if (NAIVE_LOCAL_RE.test(trimmed)) {
+    return guildLocalDateTimeToUtc(trimmed, timeZone);
+  }
+
+  const parsed = new Date(trimmed);
+  if (Number.isNaN(parsed.getTime())) {
+    throw new RangeError('invalid_when');
+  }
+  return parsed;
+}