diff --git a/apps/meteor/client/hooks/useTimezoneNameList.ts b/apps/meteor/client/hooks/useTimezoneNameList.ts
index e51d8812ac2c9..3c19cba8e56e7 100644
--- a/apps/meteor/client/hooks/useTimezoneNameList.ts
+++ b/apps/meteor/client/hooks/useTimezoneNameList.ts
@@ -1,8 +1,8 @@
+import { getTimezoneNames } from '@rocket.chat/tools';
 import { useMemo } from 'react';
 
-const getTimeZoneNames = (): string[] => {
-	const intl = Intl as typeof Intl & { supportedValuesOf?(key: 'timeZone'): string[] };
-	return typeof intl.supportedValuesOf === 'function' ? intl.supportedValuesOf('timeZone') : [];
-};
-
-export const useTimezoneNameList = (): string[] => useMemo(() => getTimeZoneNames(), []);
+export const useTimezoneNameList = (): string[] =>
+	useMemo(() => {
+		const names = getTimezoneNames();
+		return names.includes('UTC') ? names : ['UTC', ...names];
+	}, []);
diff --git a/apps/meteor/client/views/admin/settings/Setting/inputs/SelectTimezoneSettingInput.tsx b/apps/meteor/client/views/admin/settings/Setting/inputs/SelectTimezoneSettingInput.tsx
index 9db35413302f6..8008be82d8f06 100644
--- a/apps/meteor/client/views/admin/settings/Setting/inputs/SelectTimezoneSettingInput.tsx
+++ b/apps/meteor/client/views/admin/settings/Setting/inputs/SelectTimezoneSettingInput.tsx
@@ -1,4 +1,5 @@
 import { Field, FieldHint, FieldLabel, FieldRow, Select } from '@rocket.chat/fuselage';
+import { canonicalizeTimezone } from '@rocket.chat/tools';
 import type { ReactElement } from 'react';
 
 import { useTimezoneNameList } from '../../../../../hooks/useTimezoneNameList';
@@ -38,7 +39,7 @@ function SelectTimezoneSettingInput({
 			<FieldRow>
 				<Select
 					id={_id}
-					value={value}
+					value={typeof value === 'string' ? canonicalizeTimezone(value) : value}
 					placeholder={placeholder}
 					disabled={disabled}
 					readOnly={readonly}
diff --git a/apps/meteor/client/views/omnichannel/businessHours/EditBusinessHours.tsx b/apps/meteor/client/views/omnichannel/businessHours/EditBusinessHours.tsx
index 9c6ad2a986348..05288e002224b 100644
--- a/apps/meteor/client/views/omnichannel/businessHours/EditBusinessHours.tsx
+++ b/apps/meteor/client/views/omnichannel/businessHours/EditBusinessHours.tsx
@@ -1,6 +1,7 @@
 import type { ILivechatBusinessHour, LivechatBusinessHourTypes, Serialized } from '@rocket.chat/core-typings';
 import { Box, Button, ButtonGroup } from '@rocket.chat/fuselage';
 import { useEffectEvent } from '@rocket.chat/fuselage-hooks';
+import { canonicalizeTimezone } from '@rocket.chat/tools';
 import { Page, PageFooter, PageHeader, PageScrollableContentWithShadow } from '@rocket.chat/ui-client';
 import { useToastMessageDispatch, useTranslation, useRouter, useEndpoint } from '@rocket.chat/ui-contexts';
 import { useId } from 'react';
@@ -14,7 +15,7 @@ import { useRemoveBusinessHour } from './useRemoveBusinessHour';
 
 const getInitialData = (businessHourData: Serialized<ILivechatBusinessHour> | undefined) => ({
 	name: businessHourData?.name || '',
-	timezoneName: businessHourData?.timezone?.name || 'America/Sao_Paulo',
+	timezoneName: canonicalizeTimezone(businessHourData?.timezone?.name || 'America/Sao_Paulo'),
 	daysOpen: (businessHourData?.workHours || defaultWorkHours()).filter(({ open }) => !!open).map(({ day }) => day),
 	daysTime: (businessHourData?.workHours || defaultWorkHours())
 		.filter(({ open }) => !!open)
diff --git a/packages/tools/src/timezone.spec.ts b/packages/tools/src/timezone.spec.ts
new file mode 100644
index 0000000000000..7e1025674e718
--- /dev/null
+++ b/packages/tools/src/timezone.spec.ts
@@ -0,0 +1,57 @@
+import { canonicalizeTimezone, getTimezoneNames } from './timezone';
+
+describe('canonicalizeTimezone', () => {
+	it('returns the same value for a canonical IANA zone', () => {
+		expect(canonicalizeTimezone('America/Sao_Paulo')).toBe('America/Sao_Paulo');
+	});
+
+	it('resolves plain UTC to UTC', () => {
+		expect(canonicalizeTimezone('UTC')).toBe('UTC');
+	});
+
+	it('resolves Etc/UTC and Etc/GMT to UTC', () => {
+		expect(canonicalizeTimezone('Etc/UTC')).toBe('UTC');
+		expect(canonicalizeTimezone('Etc/GMT')).toBe('UTC');
+	});
+
+	it('resolves legacy moment aliases to their canonical zone', () => {
+		expect(canonicalizeTimezone('GMT')).toBe('UTC');
+		expect(canonicalizeTimezone('Zulu')).toBe('UTC');
+		expect(canonicalizeTimezone('Universal')).toBe('UTC');
+		expect(canonicalizeTimezone('US/Pacific')).toBe('America/Los_Angeles');
+		expect(canonicalizeTimezone('Japan')).toBe('Asia/Tokyo');
+	});
+
+	it('resolves legacy IANA names to modern canonical names', () => {
+		expect(canonicalizeTimezone('Asia/Calcutta')).toBe('Asia/Kolkata');
+		expect(canonicalizeTimezone('Asia/Katmandu')).toBe('Asia/Kathmandu');
+		expect(canonicalizeTimezone('Asia/Rangoon')).toBe('Asia/Yangon');
+		expect(canonicalizeTimezone('Asia/Saigon')).toBe('Asia/Ho_Chi_Minh');
+		expect(canonicalizeTimezone('Europe/Kiev')).toBe('Europe/Kyiv');
+		expect(canonicalizeTimezone('America/Godthab')).toBe('America/Nuuk');
+		expect(canonicalizeTimezone('Pacific/Enderbury')).toBe('Pacific/Kanton');
+	});
+
+	it('preserves modern canonical names as-is', () => {
+		expect(canonicalizeTimezone('Asia/Kolkata')).toBe('Asia/Kolkata');
+		expect(canonicalizeTimezone('Europe/Kyiv')).toBe('Europe/Kyiv');
+		expect(canonicalizeTimezone('Asia/Ho_Chi_Minh')).toBe('Asia/Ho_Chi_Minh');
+	});
+
+	it('returns the input unchanged when it is not a recognized zone', () => {
+		const input = 'Not/A_Zone';
+		expect(canonicalizeTimezone(input)).toBe(input);
+	});
+});
+
+describe('getTimezoneNames', () => {
+	it('returns modern canonical names instead of legacy ones', () => {
+		const names = getTimezoneNames();
+		expect(names).toContain('Asia/Kolkata');
+		expect(names).not.toContain('Asia/Calcutta');
+		expect(names).toContain('Europe/Kyiv');
+		expect(names).not.toContain('Europe/Kiev');
+		expect(names).toContain('Asia/Yangon');
+		expect(names).not.toContain('Asia/Rangoon');
+	});
+});
diff --git a/packages/tools/src/timezone.ts b/packages/tools/src/timezone.ts
index d418457d955aa..588a5df937b3c 100644
--- a/packages/tools/src/timezone.ts
+++ b/packages/tools/src/timezone.ts
@@ -1,3 +1,40 @@
+// Zones where Node/browser Intl returns a legacy IANA name instead of the
+// current canonical one. Workaround until Temporal lands (~late 2026).
+// Source: https://data.iana.org/time-zones/tzdb/backward
+// Ref: https://github.com/tc39/proposal-temporal/issues/3249
+const LEGACY_TO_CANONICAL: Record<string, string> = {
+	'America/Buenos_Aires': 'America/Argentina/Buenos_Aires',
+	'America/Catamarca': 'America/Argentina/Catamarca',
+	'America/Cordoba': 'America/Argentina/Cordoba',
+	'America/Godthab': 'America/Nuuk',
+	'America/Indianapolis': 'America/Indiana/Indianapolis',
+	'America/Jujuy': 'America/Argentina/Jujuy',
+	'America/Louisville': 'America/Kentucky/Louisville',
+	'America/Mendoza': 'America/Argentina/Mendoza',
+	'Asia/Calcutta': 'Asia/Kolkata',
+	'Asia/Katmandu': 'Asia/Kathmandu',
+	'Asia/Rangoon': 'Asia/Yangon',
+	'Asia/Saigon': 'Asia/Ho_Chi_Minh',
+	'Atlantic/Faeroe': 'Atlantic/Faroe',
+	'Europe/Kiev': 'Europe/Kyiv',
+	'Pacific/Enderbury': 'Pacific/Kanton',
+};
+
+export const canonicalizeTimezone = (name: string): string => {
+	try {
+		const resolved = new Intl.DateTimeFormat(undefined, { timeZone: name }).resolvedOptions().timeZone;
+		return LEGACY_TO_CANONICAL[resolved] ?? resolved;
+	} catch {
+		return name;
+	}
+};
+
+export const getTimezoneNames = (): string[] => {
+	const intl = Intl as typeof Intl & { supportedValuesOf?(key: 'timeZone'): string[] };
+	const zones = typeof intl.supportedValuesOf === 'function' ? intl.supportedValuesOf('timeZone') : [];
+	return zones.map((name) => LEGACY_TO_CANONICAL[name] ?? name).sort();
+};
+
 export const guessTimezoneFromOffset = (offset: string | number): string => {
 	const hours = Number(offset);
 	const totalMinutes = Math.round(hours * 60);
@@ -21,7 +58,7 @@ export const guessTimezoneFromOffset = (offset: string | number): string => {
 		const tzHours = match[1] ? parseInt(match[1], 10) : 0;
 		const tzMinutes = match[2] ? parseInt(match[2], 10) * (tzHours < 0 ? -1 : 1) : 0;
 		if (tzHours * 60 + tzMinutes === totalMinutes) {
-			return tz;
+			return LEGACY_TO_CANONICAL[tz] ?? tz;
 		}
 	}
 
@@ -33,4 +70,7 @@ export const guessTimezoneFromOffset = (offset: string | number): string => {
 	return `Etc/GMT${intHours > 0 ? '-' : '+'}${Math.abs(intHours)}`;
 };
 
-export const guessTimezone = (): string => new Intl.DateTimeFormat().resolvedOptions().timeZone;
+export const guessTimezone = (): string => {
+	const resolved = new Intl.DateTimeFormat().resolvedOptions().timeZone;
+	return LEGACY_TO_CANONICAL[resolved] ?? resolved;
+};