diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c6e2ce48..fb6f3225 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,6 +21,7 @@ defaults:
 
 env:
   TZ: 'America/Phoenix'
+  LOCALE_ID: 'en-US'
 
 jobs:
   install-and-build:
diff --git a/packages/xpath/test/helpers.ts b/packages/xpath/test/helpers.ts
index 9ce0427f..7c9bd13b 100644
--- a/packages/xpath/test/helpers.ts
+++ b/packages/xpath/test/helpers.ts
@@ -10,14 +10,33 @@ type AnyParentNode =
 	| XMLDocument;
 
 declare global {
+	/**
+	 * The timezone identifier used for all date and time operations in tests.
+	 * This string follows the IANA Time Zone Database format (e.g., 'America/Phoenix').
+	 * It determines the offset and DST behavior for `Date` objects and
+	 * related functions.
+	 *
+	 * @example 'America/Phoenix' // Fixed UTC-7, no DST
+	 * @example 'Europe/London' // UTC+0 (GMT) or UTC+1 (BST) with DST
+	 */
 	// eslint-disable-next-line no-var
 	var TZ: string | undefined;
 	// eslint-disable-next-line no-var
+	var LOCALE_ID: string | undefined;
+	/**
+	 * The locale string defining the language and regional formatting for tests.
+	 * This follows the BCP 47 language tag format (e.g., 'en-US'). It ensures consistent formatting
+	 * across tests.
+	 *
+	 * @example 'en-US' // American English
+	 */
+	// eslint-disable-next-line no-var
 	var IMPLEMENTATION: string | undefined;
 }
 
 globalThis.IMPLEMENTATION = typeof IMPLEMENTATION === 'string' ? IMPLEMENTATION : undefined;
 globalThis.TZ = typeof TZ === 'string' ? TZ : undefined;
+globalThis.LOCALE_ID = typeof LOCALE_ID === 'string' ? LOCALE_ID : undefined;
 
 const namespaces: Record<string, string> = {
 	xhtml: 'http://www.w3.org/1999/xhtml',
@@ -338,3 +357,7 @@ export const getNonNamespaceAttributes = (element: Element): readonly Attr[] =>
 
 	return attrs.filter(({ name }) => name !== 'xmlns' && !name.startsWith('xmlns:'));
 };
+
+export const getDefaultDateTimeLocale = (): string => {
+	return new Date().toLocaleString(LOCALE_ID, { timeZone: TZ });
+};
diff --git a/packages/xpath/test/setup.ts b/packages/xpath/test/setup.ts
new file mode 100644
index 00000000..b4ce741b
--- /dev/null
+++ b/packages/xpath/test/setup.ts
@@ -0,0 +1,14 @@
+import { afterEach, beforeEach } from 'vitest';
+import { vi } from 'vitest';
+import { getDefaultDateTimeLocale } from './helpers.ts';
+
+beforeEach(() => {
+	const dateOnTimezone = getDefaultDateTimeLocale();
+	vi.useFakeTimers({
+		now: new Date(dateOnTimezone).getTime(),
+	});
+});
+
+afterEach(() => {
+	vi.useRealTimers();
+});
diff --git a/packages/xpath/test/xforms/now.test.ts b/packages/xpath/test/xforms/now.test.ts
index ccc251f1..3454c064 100644
--- a/packages/xpath/test/xforms/now.test.ts
+++ b/packages/xpath/test/xforms/now.test.ts
@@ -14,8 +14,6 @@ describe('#now()', () => {
 	// > including timezone offset (i.e. not normalized to UTC) as described
 	// > under the dateTime datatype.
 	it('should return a timestamp for this instant', () => {
-		// this check might fail if run at precisely midnight ;-)
-
 		// given
 		const now = new Date();
 		const today = `${now.getFullYear()}-${(1 + now.getMonth()).toString().padStart(2, '0')}-${now
diff --git a/packages/xpath/vite.config.ts b/packages/xpath/vite.config.ts
index bc0aa8f2..bbd636b8 100644
--- a/packages/xpath/vite.config.ts
+++ b/packages/xpath/vite.config.ts
@@ -44,13 +44,26 @@ const TEST_ENVIRONMENT = BROWSER_ENABLED ? 'node' : 'jsdom';
  */
 const TEST_TIME_ZONE = 'America/Phoenix';
 
+/**
+ * The locale used for formatting dates and times in all test cases.
+ * This ensures consistent language and regional settings across tests.
+ * Currently set to 'en-US' (American English), which affects date formats
+ * (e.g., MM/DD/YYYY) and time separators.
+ *
+ * @constant
+ * @default 'en-US'
+ */
+const TEST_LOCALE = 'en-US';
+
 export default defineConfig(({ mode }) => {
 	const isTest = mode === 'test';
 
 	let timeZoneId: string | null = process.env.TZ ?? null;
+	let localeId: string | null = process.env.LOCALE_ID ?? null;
 
 	if (isTest) {
 		timeZoneId = timeZoneId ?? TEST_TIME_ZONE;
+		localeId = localeId ?? TEST_LOCALE;
 	}
 
 	// `expressionParser.ts` is built as a separate entry so it can be consumed
@@ -83,6 +96,7 @@ export default defineConfig(({ mode }) => {
 		},
 		define: {
 			TZ: JSON.stringify(timeZoneId),
+			LOCALE_ID: JSON.stringify(localeId),
 		},
 		esbuild: {
 			target: 'esnext',
@@ -122,7 +136,7 @@ export default defineConfig(({ mode }) => {
 				headless: true,
 				screenshotFailures: false,
 			},
-
+			setupFiles: ['test/setup.ts'],
 			environment: TEST_ENVIRONMENT,
 			globals: false,
 			include: ['test/**/*.test.ts'],