MUI Date Picker V7: If you're using react hook useForm you'll need to set the register function inside the textfield object of the slotProps.

<DatePicker ...
    slotProps={{
     textField: {
      ...register('propertyName'),
      error: !!errors?.propertName?.message,
      helperText: errors?.propertyName?.message
     },
    }}
...  />