refactor(date): delegate to v0 createDate as runtime

packages/docs/src/pages/en/getting-started/upgrade-guide.md

@@ -235,6 +235,54 @@ export default createVuetify({
 
 Theme colors now support transparency. `rgb(var(--v-theme-color))` will continue to work the same as before, but `rgba(var(--v-theme-color), 0.8)` should be changed to either `color-mix(in srgb, rgb(var(--v-theme-color)) 80%, transparent)` or `rgb(from rgb(var(--v-theme-color)) / 0.8)` when used with a transparent theme color.
 
+## Date
+
+The date system now uses `@vuetify/v0` under the hood. The consumer API (`useDate()`) is unchanged for most users.
+
+### Default date type changed
+
+The default adapter now uses `Temporal.PlainDateTime` instead of native `Date`. If your code handles date values from VDatePicker or VCalendar model values, update accordingly:
+
+```diff
+- const year = modelValue.getFullYear()
++ const year = modelValue.year
+```
+
+### Locale-derived week settings removed
+
+The default adapter no longer automatically derives `firstDayOfWeek` from the locale. All locales default to Sunday (0). Pass `firstDayOfWeek` explicitly to week-related methods if you need locale-specific behavior:
+
+```ts
+adapter.startOfWeek(date, 1) // Monday
+adapter.getWeekArray(date, 1)
+```
+
+### Format differences
+
+- `toISO()` now returns `"YYYY-MM-DDTHH:mm:ss"` (includes time) instead of `"YYYY-MM-DD"`
+- `getWeekdays()` defaults to `'short'` format (`Sun`, `Mon`) instead of `'narrow'` (`S`, `M`)
+- Time formats include seconds (`13:00:00` instead of `13:00`)
+
+### Custom adapter migration
+
+Custom adapters must implement `@vuetify/v0`'s `DateAdapter<T>` interface. Old adapters are auto-detected and wrapped with a deprecation warning. Key signature changes:
+
+- `getWeek(date, firstDayOfWeek?, minimalDays?)` — third parameter is now `minimalDays` (minimum days in first week) instead of `firstDayOfYear`
+- Several new methods required: `parse`, `isNull`, `formatByString`, `getFormatHelperText`, `formatNumber`, `getMeridiemText`, and others
+
+### Third-party adapters
+
+Date library adapters (luxon, dayjs, date-fns) will be shipped by `@vuetify/v0`:
+
+```diff
+- import { VuetifyLuxonAdapter } from 'vuetify/date/adapters/luxon'
++ import { LuxonDateAdapter } from '@vuetify/v0/date'
+```
+
+### Direct symbol injection deprecated
+
+Use `useDate()` instead of injecting `DateAdapterSymbol` directly.
+
 ## Components
 
 ### VBtn display

packages/vuetify/src/components/VDatePicker/__tests__/VDatePicker.date.spec.ts

@@ -800,6 +800,9 @@ describe('week numbers with time zone', () => {
 })
 
 describe('range selection with time zone', () => {
+  // Temporal.PlainDateTime is timezone-agnostic, so TZ stubbing
+  // does not affect date calculations. Tests verify correct dates
+  // are selected regardless of system timezone.
   beforeEach(() => vi.stubEnv('TZ', 'America/New_York'))
   afterEach(() => vi.unstubAllEnvs())
 
@@ -815,10 +818,20 @@ describe('range selection with time zone', () => {
 
     const btn1 = await wrapper.findByText('8') as HTMLElement
     btn1.click()
-    expect(update).toHaveBeenNthCalledWith(1, [new Date('2025-03-08T05:00:00.000Z')])
+    expect(update).toHaveBeenCalledTimes(1)
+    const first = update.mock.calls[0][0]
+    expect(first).toHaveLength(1)
+    expect(first[0].day).toBe(8)
+    expect(first[0].month).toBe(3)
+    expect(first[0].year).toBe(2025)
+
     const btn2 = await wrapper.findByText('9') as HTMLElement
     btn2.click()
-    expect(update).toHaveBeenNthCalledWith(2, [new Date('2025-03-08T05:00:00.000Z'), new Date('2025-03-10T03:59:59.999Z')])
+    expect(update).toHaveBeenCalledTimes(2)
+    const second = update.mock.calls[1][0]
+    expect(second).toHaveLength(2)
+    expect(second[0].day).toBe(8)
+    expect(second[1].day).toBe(9)
   })
 
   it('should select correct dates near DST/ST transition', async () => {
@@ -833,9 +846,19 @@ describe('range selection with time zone', () => {
 
     const btn1 = await wrapper.findByText('2') as HTMLElement
     btn1.click()
-    expect(update).toHaveBeenNthCalledWith(1, [new Date('2025-10-02T04:00:00.000Z')])
+    expect(update).toHaveBeenCalledTimes(1)
+    const first = update.mock.calls[0][0]
+    expect(first).toHaveLength(1)
+    expect(first[0].day).toBe(2)
+    expect(first[0].month).toBe(10)
+    expect(first[0].year).toBe(2025)
+
     const btn2 = await wrapper.findByText('3') as HTMLElement
     btn2.click()
-    expect(update).toHaveBeenNthCalledWith(2, [new Date('2025-10-02T04:00:00.000Z'), new Date('2025-10-04T03:59:59.999Z')])
+    expect(update).toHaveBeenCalledTimes(2)
+    const second = update.mock.calls[1][0]
+    expect(second).toHaveLength(2)
+    expect(second[0].day).toBe(2)
+    expect(second[1].day).toBe(3)
   })
 })

packages/vuetify/src/components/VDatePicker/__tests__/VDatePicker.spec.browser.tsx

@@ -21,11 +21,11 @@ describe('VDatePicker', () => {
     // Expect only 2 dates (first and last) to be selected
     await expect.poll(() => model.value).toHaveLength(2)
 
-    // Verify the correct dates are selected (10th and 20th)
-    const dates = model.value as Date[]
+    // Model emits Temporal.PlainDateTime (v0 adapter)
+    const dates = model.value as any[]
 
-    expect(dates[0].getDate()).toBe(10)
-    expect(dates[1].getDate()).toBe(20)
+    expect(dates[0].day).toBe(10)
+    expect(dates[1].day).toBe(20)
   })
 
   it('selects a range of dates across month boundary', async () => {
@@ -53,15 +53,15 @@ describe('VDatePicker', () => {
     // Expect only 2 dates (first and last) spanning across two months
     await expect.poll(() => model.value).toHaveLength(2)
 
-    // Verify the correct dates are selected (2025-01-07 and 2025-02-07)
-    const dates = model.value as Date[]
+    // Model emits Temporal.PlainDateTime (v0 adapter)
+    const dates = model.value as any[]
 
-    expect(dates[0].getFullYear()).toBe(2025)
-    expect(dates[0].getMonth()).toBe(0)
-    expect(dates[0].getDate()).toBe(7)
-    expect(dates[1].getFullYear()).toBe(2025)
-    expect(dates[1].getMonth()).toBe(1)
-    expect(dates[1].getDate()).toBe(7)
+    expect(dates[0].year).toBe(2025)
+    expect(dates[0].month).toBe(1) // Temporal months are 1-based
+    expect(dates[0].day).toBe(7)
+    expect(dates[1].year).toBe(2025)
+    expect(dates[1].month).toBe(2) // Temporal months are 1-based
+    expect(dates[1].day).toBe(7)
   })
 
   it('does not trigger infinite loop when first-day-of-week is out of range', async () => {