fix: track SvelteDate snapshots in reactions (#18700)

Read the Date value through `getTime()` before cloning it to get reactive updates from `SvelteDate`.

Closes #18698

---------

Co-authored-by: svelte-triage-bot <team@svelte.com>
Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/calm-dates-track.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: track SvelteDate snapshots in reactions

packages/svelte/src/internal/shared/clone.js

@@ -105,6 +105,8 @@ function clone(value, cloned, path, paths, original = null, no_tojson = false) {
 		}
 
 		if (value instanceof Date) {
+			// Ensure SvelteDate snapshots are tracked
+			value.getTime();
 			return /** @type {Snapshot<T>} */ (structuredClone(value));
 		}
 

packages/svelte/tests/runtime-runes/samples/state-snapshot-svelte-date/_config.js

@@ -0,0 +1,13 @@
+import { test } from '../../test';
+import { tick } from 'svelte';
+
+export default test({
+	async test({ assert, target }) {
+		assert.htmlEqual(target.innerHTML, '<button>update</button><p>1970-01-01T00:00:00.000Z</p>');
+
+		target.querySelector('button')?.click();
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, '<button>update</button><p>1970-01-02T00:00:00.000Z</p>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/state-snapshot-svelte-date/main.svelte

@@ -0,0 +1,9 @@
+<script>
+	import { SvelteDate } from 'svelte/reactivity';
+
+	const date = new SvelteDate(0);
+	const snapshot = $derived($state.snapshot(date));
+</script>
+
+<button onclick={() => date.setTime(86400000)}>update</button>
+<p>{snapshot.toISOString()}</p>