fix(VDatePickerMonth): range bug with programmatic startdate

fix(VDatePickerMonth): range bug with programmatic startdate
resolves vuetifyjs#20168

packages/vuetify/src/components/VDatePicker/VDatePickerMonth.tsx

@@ -11,8 +11,8 @@ import { useDate } from '@/composables/date/date'
 import { MaybeTransition } from '@/composables/transition'
 
 // Utilities
-import { computed, ref, shallowRef, watch } from 'vue'
 import { genericComponent, propsFactory } from '@/util'
+import { computed, ref, shallowRef, watch } from 'vue'
 
 // Types
 import type { PropType } from 'vue'
@@ -94,6 +94,9 @@ export const VDatePickerMonth = genericComponent<VDatePickerMonthSlots>()({
       if (model.value.length === 0) {
         rangeStart.value = undefined
       }
+      if (model.value.length > 0 && !rangeStart.value) {
+        rangeStart.value = model.value[0]
+      }
       if (!rangeStart.value) {
         rangeStart.value = _value
         model.value = [rangeStart.value]