Add publish_date and expiry_date support

docs/04-development/02-content-system.md

@@ -55,9 +55,44 @@ Returns flat key-value array with a special `_raw` key containing the full parse
 | `redirect` | string | — | External URL (list items can redirect) |
 | `feed` | bool | `false` | Enable Atom feed on list pages (`feed.xml`) |
 | `author` | string | title | Atom feed author name (falls back to page title) |
+| `publish_date` | string (YYYY-MM-DD) | — | Earliest date content is visible |
+| `expiry_date` | string (YYYY-MM-DD) | — | Last date content is visible (inclusive) |
 | `plugins` | string | — | Comma-separated page-level plugin names |
 
-### Settings Section
+## Visibility (Scheduling)
+
+Content can be scheduled using `publish_date` and `expiry_date`. Both fields are optional — when absent, content is always visible.
+
+When set:
+- **`publish_date`** — Content becomes visible on this date (inclusive). Before this date it is treated as non-existent.
+- **`expiry_date`** — Content remains visible through this date (inclusive). After this date it is treated as non-existent.
+
+Invisible content is:
+- Hidden from list views (`buildListItems()`)
+- Excluded from navigation (`buildNavigation()`)
+- Returns 404 on direct access (page and list cases in `router.php`)
+- Omitted from Atom feeds automatically (feeds use the filtered items array)
+
+The check uses UTC (`gmdate('Y-m-d')`) so behavior is consistent regardless of server timezone.
+
+These fields can be overridden per-language in `[en]` sections via the language plugin:
+
+```ini
+title = "Event"
+publish_date = "2025-10-01"
+expiry_date  = "2025-10-07"
+
+[no]
+title = "Arrangement"
+publish_date = "2025-10-01"
+expiry_date  = "2025-10-08"
+```
+
+### `isVisible(?array $metadata): bool`
+
+Pure function in `helpers.php`. Returns `true` when content should be visible, `false` otherwise. Accepts nullable metadata — returns `true` for `null` (no metadata file = always visible).
+
+## Settings Section
 
 ```ini
 [settings]