How to set date validator end_date must be after start date 7 months or (212 days)

Try

        $startDate = Carbon::createFromFormat('Y-m-d', $request->input('start_date'));
        $endDate = $startDate->addMonths(7)->toDateString();

        $validator = Validator::make([
            'start_date' => 'required|date',
            'end_date' => 'required|date|after:'.$endDate,
        ]);
        // or
        $validator = Validator::make([
            'start_date' => 'required|date',
            'end_date' => 'required|date|after_or_equal:'.$endDate,
        ]);