diff --git a/docs/api/admin.md b/docs/api/admin.md
index 445e7ce..93d388c 100644
--- a/docs/api/admin.md
+++ b/docs/api/admin.md
@@ -46,9 +46,11 @@ removes the row.
 * `logo` up to 2048 chars (a public URL — uploaded separately)
 * `welcomeMarkdown` up to 64 000 chars
 * `themeKey` is one of `THEME_IDS`
-* `eventStartUtc` / `eventEndUtc` parseable dates; `superRefine`
-  enforces `eventEndUtc > eventStartUtc` and reports the issue on
-  `eventEndUtc`
+* `eventStartUtc` / `eventEndUtc` are validated as ISO-8601 datetimes
+  (zod `string().datetime(...)`) — empty strings are explicitly allowed
+  to keep the event window unconfigured. `superRefine` then enforces
+  `eventEndUtc > eventStartUtc` and reports the issue on `eventEndUtc`
+  when both fields are non-empty.
 * `defaultChallengeIp` 1–255 chars
 * `registrationsEnabled` boolean
 
diff --git a/docs/architecture/key-files.md b/docs/architecture/key-files.md
index 283b5da..8f7484f 100644
--- a/docs/architecture/key-files.md
+++ b/docs/architecture/key-files.md
@@ -25,6 +25,7 @@ timestamp: 2026-07-22T13:40:00Z
 | `backend/src/modules/auth/auth.controller.ts` | Registers authentication and account endpoints. |
 | `backend/src/modules/auth/auth.service.ts` | Handles sessions, authentication, registration, and password changes. |
 | `backend/src/modules/uploads/uploads.controller.ts` | Registers admin-only multipart uploads, including Sharp-backed site-logo format validation. |
+| `backend/src/modules/admin/dto/general.dto.ts` | Zod schema for `PUT /api/v1/admin/general/settings` — string-length rules, theme-key enum, ISO-8601 datetime check on the event-window fields (empty string allowed), and the `endAfterStart` super-refine. |
 
 # Frontend
 
@@ -45,6 +46,8 @@ timestamp: 2026-07-22T13:40:00Z
 | `frontend/src/app/features/shell/header/shell-header.component.ts` | Shell title, status LED (size/shape only — color comes from the pure map via `[style.background-color]`), countdown, and user menu. |
 | `frontend/src/app/features/shell/tabs/quick-tabs.component.ts` | Main shell navigation tabs. |
 | `frontend/src/app/features/shell/change-password/change-password-modal.component.ts` | Change-password form modal. |
+| `frontend/src/app/features/admin/general.component.ts` | `AdminGeneralComponent` reactive form for `/admin/general` — per-field inline error rendering (page-title + event-start + event-end), logo upload wiring, welcome Markdown preview, event-state derivation, and SSE `general` event handling. |
+| `frontend/src/app/features/admin/general.pure.ts` | Pure helpers used by `AdminGeneralComponent` — `deriveEventState`, `endAfterStartValidator`, `normalizePageTitle`, `toIsoUtc`, `toDatetimeLocal`, `pageTitleError` / `pageTitleMessage`, `isoDatetimeValidator`, and `datetimeMessage`. |
 | `tests/frontend/authenticated-event-source.spec.ts` | Tests SSE authorization, frame transport behavior, and the `401`/`403` unauthorized path. |
 | `tests/frontend/auth-session-events.spec.ts` | Pure tests for cross-tab invalidation message encoding, payload validation, and `storage`-event filtering. |
 
diff --git a/docs/guides/admin-general-settings.md b/docs/guides/admin-general-settings.md
index d2a6ccb..39cdc30 100644
--- a/docs/guides/admin-general-settings.md
+++ b/docs/guides/admin-general-settings.md
@@ -2,8 +2,8 @@
 type: guide
 title: Admin — General Settings
 description: How an admin edits global platform settings (page title, logo, theme, event window, default challenge IP, registrations, welcome Markdown) from the /admin/general page.
-tags: [guide, admin, settings, general, tester]
-timestamp: 2026-07-22T12:44:45Z
+tags: [guide, admin, settings, general, tester, datetime, validation]
+timestamp: 2026-07-22T14:07:58Z
 ---
 
 # When this view is available
@@ -42,8 +42,8 @@ The page is a single reactive form with these controls (every
 | Logo (file picker)     | `general-logo-file`       | `logo` (public URL)     | Uploads via `POST /api/v1/uploads/logo`; the returned `publicUrl` is bound to a hidden input `general-logo`. |
 | Logo upload status     | `general-logo-uploading` / `general-logo-error` / `general-logo-current` | — | Inline status text under the file picker. |
 | Global theme           | `general-themeKey`        | `themeKey`              | `<select>` populated from `/themes`. Value is one of `THEME_IDS`. |
-| Event start (UTC)      | `general-eventStart`      | `eventStartUtc`         | `datetime-local` input converted to ISO UTC on save. |
-| Event end (UTC)        | `general-eventEnd`        | `eventEndUtc`           | Must be strictly after Event start. Invalid pair renders `general-endBeforeStart`. |
+| Event start (UTC)      | `general-eventStart`      | `eventStartUtc`         | `datetime-local` input. Empty string is allowed; any non-empty value must be a valid ISO-8601 datetime — invalid input renders `general-eventStart-error`. |
+| Event end (UTC)        | `general-eventEnd`        | `eventEndUtc`           | `datetime-local` input. Empty string is allowed; any non-empty value must be a valid ISO-8601 datetime — invalid input renders `general-eventEnd-error`. Must also be strictly after Event start (`general-endBeforeStart`). |
 | Default challenge IP   | `general-defaultIp`       | `defaultChallengeIp`    | Required, max 255 chars. |
 | Enable registrations   | `general-registrations`   | `registrationsEnabled`  | Boolean checkbox. When `false`, the public register endpoint returns `REGISTRATIONS_DISABLED`. |
 | Welcome description    | `general-welcome`         | `welcomeMarkdown`       | Multi-line textarea; a live preview is rendered into `general-welcome-preview`. |
@@ -74,6 +74,15 @@ The page is a single reactive form with these controls (every
   strictly after the start, the form becomes invalid and
   `general-endBeforeStart` appears under the end input. Save remains
   disabled.
+* **Per-field datetime validation:** if the user types (or leaves) a value
+  that is not parseable as an ISO-8601 datetime in either the Event start
+  or Event end input, the affected input flips `aria-invalid="true"` and
+  its inline error element (`general-eventStart-error` or
+  `general-eventEnd-error`) renders the message
+  `"<Field> must be a valid ISO-8601 datetime."`. Empty strings are
+  considered valid (the event window may be unconfigured), so clearing
+  both inputs removes the error. Save remains disabled while either input
+  is invalid.
 * **Save:** clicking Save sends `PUT /api/v1/admin/general/settings`
   with all fields. The Page title input is validated client-side for
   non-blank content; an empty or whitespace-only Page title disables the
@@ -117,6 +126,50 @@ Save, the control is reset to untouched/pristine and the
 `pageTitleTouchedOrDirty` signal is cleared, so reloading valid
 settings does not flash a stale error.
 
+# Event start / end inline error messages
+
+The inline error elements under the Event start and Event end inputs
+(`general-eventStart-error` and `general-eventEnd-error`) are driven by
+two pure helpers exported from
+`frontend/src/app/features/admin/general.pure.ts`:
+
+* `isoDatetimeValidator(ctrl)` — an Angular reactive-forms validator
+  that returns `null` for an empty value and `{ invalidDatetime: true }`
+  for any non-empty value that `Date.parse` cannot parse. Empty is
+  intentionally valid so the event window can be cleared.
+* `datetimeMessage(fieldLabel, value, controlErrors)` — renders the
+  human-readable message `"<Field> must be a valid ISO-8601 datetime."`
+  either when `controlErrors.invalidDatetime` is set, or as a fallback
+  when the raw value itself is non-empty and unparseable.
+
+The component wires both controls (`eventStartUtc`, `eventEndUtc`) with
+this validator and keeps three local `signal`s per field — `Value`,
+`Invalid`, `TouchedOrDirty` — exactly mirroring the Page-title pattern.
+The `computed` signals `showEventStartError` / `showEventEndError` and
+`eventStartMessage` / `eventEndMessage` are then bound to:
+
+* `[attr.aria-invalid]` and `[attr.aria-describedby]` on each input so
+  assistive tech is informed the instant the field goes invalid.
+* An `@if` block that renders the inline `<div class="field-error">`
+  with `data-testid="general-eventStart-error"` /
+  `data-testid="general-eventEnd-error"`.
+
+| Condition                                                  | Message                                                |
+|------------------------------------------------------------|--------------------------------------------------------|
+| Empty value (`''`)                                         | `null` (no error; the event window may be unconfigured). |
+| Non-empty value that `Date.parse` accepts                 | `null` (no error).                                     |
+| Non-empty value that `Date.parse` rejects                 | `"<Field> must be a valid ISO-8601 datetime."`        |
+| Control carries `invalidDatetime` error                   | Same message (covered by the validator path).          |
+
+The error is shown only when the control is both `touched || dirty`
+AND `invalid`, so the inputs do not flash errors on initial load. After
+a successful Save, both controls are reset to untouched/pristine via
+`resetTouchedState()` and their `TouchedOrDirty` signals are cleared.
+The cross-field `endBeforeStart` error (`general-endBeforeStart`) is
+still emitted from the form-level `superRefine`/validator chain and is
+unchanged — it now sits next to the new per-field messages when the
+user supplies two non-empty values that are out of order.
+
 # Visual elements
 
 | Element                       | Selector                                            |
@@ -127,6 +180,8 @@ settings does not flash a stale error.
 | Form                          | `[data-testid="general-form"]`                      |
 | Welcome preview               | `[data-testid="general-welcome-preview"]`           |
 | End-before-start message      | `[data-testid="general-endBeforeStart"]`            |
+| Event start inline error      | `[data-testid="general-eventStart-error"]`          |
+| Event end inline error        | `[data-testid="general-eventEnd-error"]`            |
 | Page-title inline error       | `[data-testid="general-pageTitle-error"]`           |
 
 # Architecture map
@@ -138,7 +193,7 @@ settings does not flash a stale error.
 | 3    | `frontend/src/app/core/services/admin.service.ts`    | `getGeneralSettings()` → `GET /api/v1/admin/general/settings`; `listAdminThemes()` → `GET /api/v1/admin/general/themes`; `updateGeneralSettings()` → `PUT .../settings`; `uploadLogo()` → `POST /api/v1/uploads/logo`. |
 | 4    | `backend/src/modules/admin/admin-general.controller.ts` | `AdminGuard` + `@Roles('admin')` on every handler.                       |
 | 5    | `backend/src/modules/admin/general.service.ts`       | `getSettings` reads 8 keys via `SettingsService`; `updateSettings` writes all 8, emits `{ topic: 'general', themeKey }` via `SseHubService`. |
-| 6    | `backend/src/modules/admin/dto/general.dto.ts`       | `GeneralSettingsSchema` enforces string lengths, `themeKey` enum, and `eventEndUtc > eventStartUtc` via `superRefine`. |
+| 6    | `backend/src/modules/admin/dto/general.dto.ts`       | `GeneralSettingsSchema` enforces string lengths, `themeKey` enum, per-field ISO-8601 datetime check on `eventStartUtc`/`eventEndUtc` (empty string allowed), and `eventEndUtc > eventStartUtc` via `superRefine`. |
 
 # Notes