fix: ensure SvelteDate cached methods have correct reactive context

.changeset/wicked-zebras-exist.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure SvelteDate cached methods have correct reactive context

packages/svelte/src/internal/client/dom/elements/bindings/shared.js

@@ -34,6 +34,7 @@ export function listen(target, events, handler, call_handler_immediately = true)
 /**
  * @template T
  * @param {() => T} fn
+ * @returns {T}
  */
 export function without_reactive_context(fn) {
 	var previous_reaction = active_reaction;

packages/svelte/src/reactivity/date.js

@@ -1,11 +1,35 @@
-/** @import { Source } from '#client' */
-import { DESTROYED } from '../internal/client/constants.js';
+/** @import { Source, Derived, Effect } from '#client' */
+import { DERIVED } from '../internal/client/constants.js';
 import { derived } from '../internal/client/index.js';
 import { source, set } from '../internal/client/reactivity/sources.js';
-import { get } from '../internal/client/runtime.js';
+import { active_reaction, get, set_active_reaction } from '../internal/client/runtime.js';
 
 var inited = false;
 
+/**
+ * @template T
+ * @param {() => T} fn
+ * @returns {T}
+ */
+function with_parent_effect(fn) {
+	var previous_reaction = active_reaction;
+	var parent_reaction = active_reaction;
+
+	while (parent_reaction !== null) {
+		if ((parent_reaction.f & DERIVED) === 0) {
+			break;
+		}
+		parent_reaction = /** @type {Derived | Effect} */ (parent_reaction).parent;
+	}
+
+	set_active_reaction(parent_reaction);
+	try {
+		return fn();
+	} finally {
+		set_active_reaction(previous_reaction);
+	}
+}
+
 export class SvelteDate extends Date {
 	#time = source(super.getTime());
 
@@ -43,12 +67,17 @@ export class SvelteDate extends Date {
 
 					var d = this.#deriveds.get(method);
 
-					if (d === undefined || (d.f & DESTROYED) !== 0) {
-						d = derived(() => {
-							get(this.#time);
-							// @ts-ignore
-							return date_proto[method].apply(this, args);
-						});
+					if (d === undefined) {
+						// Ensure we create the derived inside the nearest parent effect if
+						// we're inside a derived, otherwise the derived will be destroyed each
+						// time it re-runs
+						d = with_parent_effect(() =>
+							derived(() => {
+								get(this.#time);
+								// @ts-ignore
+								return date_proto[method].apply(this, args);
+							})
+						);
 
 						this.#deriveds.set(method, d);
 					}

packages/svelte/src/reactivity/date.test.ts

@@ -642,3 +642,33 @@ test('Date methods invoked for the first time in a derived', () => {
 
 	cleanup();
 });
+
+test('Date methods shared between deriveds', () => {
+	const date = new SvelteDate(initial_date);
+	const log: any = [];
+
+	const cleanup = effect_root(() => {
+		const year = derived(() => {
+			return date.getFullYear();
+		});
+		const year2 = derived(() => {
+			return date.getTime(), date.getFullYear();
+		});
+
+		render_effect(() => {
+			log.push(get(year) + '/' + get(year2).toString());
+		});
+
+		flushSync(() => {
+			date.setFullYear(date.getFullYear() + 1);
+		});
+
+		flushSync(() => {
+			date.setFullYear(date.getFullYear() + 1);
+		});
+	});
+
+	assert.deepEqual(log, ['2023/2023', '2024/2024', '2025/2025']);
+
+	cleanup();
+});