public void hesapla(String tarih1,String tarih2)
    {
        Calendar cal = Calendar.getInstance();
        Calendar cal2 = Calendar.getInstance();
        SimpleDateFormat sdf = new SimpleDateFormat("dd.MM.yyyy", Locale.US);
        try {
            cal.setTime(sdf.parse(tarih1));
            cal2.setTime(sdf.parse(tarih2));
        } catch (ParseException e) {
            e.printStackTrace();
        }

        Date d1 = new Date(cal.getTimeInMillis());
        Date d2 = new Date(cal2.getTimeInMillis());

        long diff = d2.getTime()- d1.getTime() ;
        String days =""+diff / (1000*60*60*24);

        Toast t =  Toast.makeText(this, days,Toast.LENGTH_LONG);
        t.setGravity(Gravity.TOP|Gravity.CENTER_HORIZONTAL,0,0);
        t.show();
    }

Kullanımı: hesapla("27.07.2018","27.08.2018");