Since Nova 1.1.8 this feature has been adopted to the core. You should upgrade Nova and use that filter instead.
Nova filter that displays a Date Picker instead of a select.
Run this command in your nova project:
composer require 64robots/nova-date-filter
Just use DateFilter class instead of Filter
use R64\Filters\DateFilter;
class DateFrom extends DateFilter
{
  //
}As Date Filter is not a select anymore we can use options method to pass the date picker config
use R64\Filters\DateFilter;
class DateFrom extends DateFilter
{
  //...
  public function options(Request $request)
  {
    return [
      'dateFormat' => 'Y-m-d', // default Y-m-d H:i:S
      'placeholder' => 'My placeholder', // default __('Pick a date')
      'disabled' => true, // default false
      'twelveHourTime' => true, // default false
      'enableTime' => true, // default false
      'enableSeconds' => true, // default false
    ];
  }
}