diff --git a/src/components/Proposal/ProposalResponseMatrix.vue b/src/components/Proposal/ProposalResponseMatrix.vue
index fff643fb4f..94adba676c 100644
--- a/src/components/Proposal/ProposalResponseMatrix.vue
+++ b/src/components/Proposal/ProposalResponseMatrix.vue
@@ -201,7 +201,7 @@ export default {
 			const groups = {}
 			dates.forEach((d) => {
 				// Apply timezone offset for grouping by day
-				const offset = getTimezoneOffset(d.date.toISOString(), this.timezoneId)
+				const offset = getTimezoneOffset(d.date, this.timezoneId)
 				const key = moment(d.date).utcOffset(offset).format('YYYY-MM-DD')
 				if (!groups[key]) {
 					groups[key] = []
@@ -209,7 +209,7 @@ export default {
 				groups[key].push(d)
 			})
 			return Object.entries(groups).map(([key, grp]: [string, ProposalDate[]]) => {
-				const offset = getTimezoneOffset(grp[0].date.toISOString(), this.timezoneId)
+				const offset = getTimezoneOffset(grp[0].date, this.timezoneId)
 				return {
 					key,
 					label: moment(grp[0].date).utcOffset(offset).format('dddd, MMMM Do'),
@@ -230,7 +230,7 @@ export default {
 		t,
 
 		dateTimeSpan(date) {
-			const offset = getTimezoneOffset(date.toISOString(), this.timezoneId)
+			const offset = getTimezoneOffset(date, this.timezoneId)
 			const startDate = moment(date).utcOffset(offset)
 			const endDate = moment(date).utcOffset(offset).add(this.proposal.duration, 'minutes')
 
@@ -255,7 +255,7 @@ export default {
 				return ''
 			}
 			// Apply timezone offset and format very compact: "7/8 2PM"
-			const offset = getTimezoneOffset(date.toISOString(), this.timezoneId)
+			const offset = getTimezoneOffset(date, this.timezoneId)
 			const adjustedDate = moment(date).utcOffset(offset)
 			return adjustedDate.format('M/D LT').replace(':00', '').replace(' ', ' ')
 		},
diff --git a/src/services/timezoneOffsetService.js b/src/services/timezoneOffsetService.js
deleted file mode 100644
index 44f8d1e64a..0000000000
--- a/src/services/timezoneOffsetService.js
+++ /dev/null
@@ -1,52 +0,0 @@
-/**
- * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
- * SPDX-License-Identifier: AGPL-3.0-or-later
- */
-
-const dtfCache = new Map()
-
-export function getTimezoneOffset(proposalDate, timezoneId) {
-	try {
-		const date = new Date(proposalDate)
-		if (isNaN(date)) {
-			return null
-		}
-
-		let dtf = dtfCache.get(timezoneId)
-		if (!dtf) {
-			dtf = new Intl.DateTimeFormat('en-US', {
-				timeZone: timezoneId,
-				hour12: false,
-				year: 'numeric',
-				month: '2-digit',
-				day: '2-digit',
-				hour: '2-digit',
-				minute: '2-digit',
-				second: '2-digit',
-			})
-			dtfCache.set(timezoneId, dtf)
-		}
-
-		const parts = dtf.formatToParts(date)
-		const values = {}
-
-		for (const { type, value } of parts) {
-			if (type !== 'literal') {
-				values[type] = value
-			}
-		}
-
-		const asUTC = Date.UTC(
-			Number(values.year),
-			Number(values.month) - 1,
-			Number(values.day),
-			Number(values.hour),
-			Number(values.minute),
-			Number(values.second),
-		)
-
-		return Math.floor((asUTC - date.getTime()) / 60000)
-	} catch {
-		return null
-	}
-}
diff --git a/src/services/timezoneOffsetService.ts b/src/services/timezoneOffsetService.ts
new file mode 100644
index 0000000000..c60bfb9968
--- /dev/null
+++ b/src/services/timezoneOffsetService.ts
@@ -0,0 +1,42 @@
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+export function getTimezoneOffset(proposalDate: Date, timezoneId: string): number {
+	let timezoneOffset = 0
+
+	try {
+		const dtf = new Intl.DateTimeFormat('en-US', {
+			timeZone: timezoneId,
+			hour12: false,
+			year: 'numeric',
+			month: '2-digit',
+			day: '2-digit',
+			hour: '2-digit',
+			minute: '2-digit',
+			second: '2-digit',
+		})
+
+		const parts = dtf.formatToParts(proposalDate)
+
+		const values: Record<string, string> = {}
+		parts.forEach(({ type, value }) => {
+			if (type !== 'literal') values[type] = value
+		})
+
+		const asUTC = Date.UTC(
+			Number(values.year),
+			Number(values.month) - 1,
+			Number(values.day),
+			Number(values.hour),
+			Number(values.minute),
+			Number(values.second),
+		)
+
+		timezoneOffset = Math.round((asUTC - proposalDate.getTime()) / 60000)
+	} catch (e) {
+		timezoneOffset = 0
+	}
+
+	return timezoneOffset
+}