diff --git a/src/components/NcDateTimePicker/NcDateTimePicker.vue b/src/components/NcDateTimePicker/NcDateTimePicker.vue
index b8e091363f..3dc890eaf7 100644
--- a/src/components/NcDateTimePicker/NcDateTimePicker.vue
+++ b/src/components/NcDateTimePicker/NcDateTimePicker.vue
@@ -295,6 +295,7 @@ import NcTimezonePicker from '../NcTimezonePicker/NcTimezonePicker.vue'
 import { t } from '../../l10n.ts'
 import NcButton from '../NcButton/index.ts'
 import { getDateFormat, getDateTimeFormat, getMonthFormat, getTimeFormat, getWeekFormat, getYearFormat } from './format.ts'
+import { checkForNonStandardTokens } from './formatValidation.ts'
 import useDateFnsLocale from './useDateFnsLocale.ts'
 
 type LibraryFormatOptions = VueDatePickerProps['format']
@@ -556,6 +557,20 @@ watch(dateFnsLocale, () => {
 	flush: 'post',
 })
 
+watchEffect(() => {
+	if (typeof props.format !== 'string') {
+		return
+	}
+	const nonStandardTokens = checkForNonStandardTokens(props.format)
+	if (nonStandardTokens.length !== 0) {
+		let warning = `[NcDateTimePicker] The \`format\` property value "${props.format}" uses the non-standard formating tokens ${nonStandardTokens.join(', ')}.`
+		warning += ' They will be disabled in the future because they are only supported by the date-fns library.'
+		warning += ' Only use tokens from the Unicode Technical Standard #35.'
+		warning += ' See https://github.com/nextcloud-libraries/nextcloud-vue/issues/8931'
+		warn(warning)
+	}
+})
+
 /**
  * The date (time) formatting to be used by the library.
  * We use the provided format if possible, otherwise we provide a localized formatting
diff --git a/src/components/NcDateTimePicker/formatValidation.ts b/src/components/NcDateTimePicker/formatValidation.ts
new file mode 100644
index 0000000000..f2fc09259e
--- /dev/null
+++ b/src/components/NcDateTimePicker/formatValidation.ts
@@ -0,0 +1,42 @@
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+/**
+ * List of tokens supported by date-fns but not Unicode Technical Standard #35.
+ *
+ * @see https://date-fns.org/v4.4.0/docs/format
+ */
+const NON_STANDARD_TOKENS_OF_DATE_FNS = new Set([
+	'i',
+	'I',
+	'R',
+	't',
+	'T',
+	'o',
+	'P',
+	'p',
+])
+
+const QUOTE_CHAR = "'"
+
+/**
+ * Returns array of used non-standard tokens.
+ *
+ * @param format The format string.
+ */
+export function checkForNonStandardTokens(format: string): string[] {
+	const usedNonStandardTokens = new Set<string>()
+	let isQuoted = false
+	for (const char of format) {
+		if (char === QUOTE_CHAR) {
+			isQuoted = !isQuoted
+		} else if (!isQuoted) {
+			if (NON_STANDARD_TOKENS_OF_DATE_FNS.has(char)) {
+				usedNonStandardTokens.add(char)
+			}
+		}
+	}
+	return [...usedNonStandardTokens]
+}
diff --git a/tests/unit/components/NcDateTimePicker/NcDateTimePicker.spec.js b/tests/unit/components/NcDateTimePicker/NcDateTimePicker.spec.js
index d8cbf5f473..05933e0f72 100644
--- a/tests/unit/components/NcDateTimePicker/NcDateTimePicker.spec.js
+++ b/tests/unit/components/NcDateTimePicker/NcDateTimePicker.spec.js
@@ -102,5 +102,49 @@ describe('NcDateTimePicker.vue', () => {
 
 			expect(wrapper.find('[data-test-id="open-time-picker-btn"]').exists()).toBe(isRendered)
 		})
+
+		describe('Non-standard token handling', () => {
+			it('warns about non-standard formatting tokens', () => {
+				const warnHandler = vi.fn()
+
+				mount(NcDateTimePicker, {
+					props: { format: 'Pp' },
+					global: {
+						config: { warnHandler },
+					},
+				})
+
+				expect(warnHandler).toHaveBeenCalledOnce()
+				expect(warnHandler.mock.calls[0][0]).toEqual('[NcDateTimePicker] The `format` property value "Pp" uses the non-standard formating tokens P, p. They will be disabled in the future because they are only supported by the date-fns library. Only use tokens from the Unicode Technical Standard #35. See https://github.com/nextcloud-libraries/nextcloud-vue/issues/8931')
+			})
+
+			it('warns about non-standard formatting tokens after property change', async () => {
+				const warnHandler = vi.fn()
+				const wrapper = mount(NcDateTimePicker, {
+					props: {
+						format: 'HH',
+					},
+					global: {
+						config: { warnHandler },
+					},
+				})
+
+				await wrapper.setProps({ format: 'II-RR' })
+
+				expect(warnHandler).toHaveBeenCalledOnce()
+				expect(warnHandler.mock.calls[0][0])
+					.toContain('[NcDateTimePicker] The `format` property value "II-RR" uses the non-standard formating tokens I, R.')
+			})
+
+			it('supports non-standard tokens', async () => {
+				const wrapper = mount(NcDateTimePicker, {
+					props: { modelValue: new Date(2026, 1, 10), format: 'II-RR' },
+				})
+
+				await nextTick()
+
+				expect(wrapper.find('input').element.value).toBe('07-2026')
+			})
+		})
 	})
 })
diff --git a/tests/unit/components/NcDateTimePicker/formatValidation.spec.ts b/tests/unit/components/NcDateTimePicker/formatValidation.spec.ts
new file mode 100644
index 0000000000..c9d7f4caf1
--- /dev/null
+++ b/tests/unit/components/NcDateTimePicker/formatValidation.spec.ts
@@ -0,0 +1,31 @@
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import { describe, expect, it } from 'vitest'
+import { checkForNonStandardTokens } from '../../../../src/components/NcDateTimePicker/formatValidation.ts'
+
+describe('checkForNonStandardTokens', () => {
+	it.for([
+		"''",
+		"h 'o''clock'",
+		"yyyy''",
+		'hh@mm',
+	])('detects in %s no non-standard tokens', (format) => {
+		const tokens = checkForNonStandardTokens(format)
+
+		expect(tokens).toHaveLength(0)
+	})
+
+	const casesWithNonStandardTokens = [
+		['II-RR', ['I', 'R']],
+		['iIRtToPp', ['i', 'I', 'R', 't', 'T', 'o', 'P', 'p']],
+	] as const
+
+	it.for(casesWithNonStandardTokens)('detects in %s non-standard tokens %j ', ([format, nonStandardTokens]) => {
+		const tokens = checkForNonStandardTokens(format)
+
+		expect(tokens).toEqual(nonStandardTokens)
+	})
+})