testFiddle

Just test

by OVK2015

HTML

<script src="https://getfirebug.com/firebug-lite-debug.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/1.12.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.11.2/moment.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-datepicker/1.6.0/js/bootstrap-datepicker.min.js"></script>
<div class="col-lg-4 col-lg-offset-4">
    <h2 class="text-center">Калькулятор оплаты</h2>
    <table class="table table-bordered">
    <tr>
    <td><p class="text-right form-control-static">На счету абонента, руб.:</p></td>
    <td><input class="form-control" type="number" id="currentBalance" value="0" placeholder="укажите сумму"/></td>
    </tr>
    <tr>
    <td><p class="text-right form-control">Сегодня:</p></td>
    <td><input id="currentDay" type="text" class="form-control"/></td>
    </tr>
    <tr>
    <td><p class="form-control" >До какого числа желают оплатить?</p></td>
    <td>
      <input id="targetDay" type="text" class="form-control">
    </td>
    <tr>
    <td><p class="text-right form-control-static">Тариф абонента:</p></td>
    <td>
      <select class="form-control" id="tarif">
        <option value="0">Тариф "Мини"</option>
        <option value="1">Тариф "Безлим 4"</option>
        <option value="2">Тариф "Безлим 10"</option>
        <option value="3">Тариф "Безлим 35"</option>
         <option value="4">Тариф "Безлим 100"</option>
        <option value="5">Тариф "Jump 5"</option>
        <option value="6">Тариф "Jump 10"</option>
        <option value="7">Тариф "Jump 35"</option>
        <option value="8">Тариф "Jump 70"</option>
      </select>
    </td>
    </tr>
    <tr>
    <td><p class="text-right form-control-static">К оплате, руб:</p></td>
    <td><input class="form-control" type="text" id="sum" readonly/></td>
    </tr>
    <tr>
    <td colspan="2" class="text-right">
    <button class="btn btn-info">Поехали!</button>
    </td>
    </tr>
    </table>

JavaScript

var tarifs = new Array(150, 225, 261, 297, 378, 150+57.60, 225+54, 261+48, 297+42, 378+18, 306, 360, 405, 522, 57.60);	

function evalResult()
{ 
      // var payByMonth = tarifs[$('.form-control option:selected').val()];
    var startDateInterval = moment($('#currentDay').val(), 'DD.MM.YYYY');
    var endDateInterval = moment($('#targetDay').val(), 'DD.MM.YYYY');
      // console.log('Начало интервала: ' + startDateInterval.format('YYYY-MM-DD'));
      // console.log('Окончание интервала: ' + endDateInterval.format('YYYY-MM-DD'));

      var currentBalance;
      if(($('#currentBalance').val() == '') || (parseFloat($('#currentBalance').val()) == NaN))
      {
        currentBalance = 0;
      }
      else
      {
       currentBalance = parseFloat($('#currentBalance').val());
      }

    var summaP;
    if(($('#summa2').val() == '') || (parseFloat($('#summa2').val()) == NaN))
      {
        summaP = 0;
      }
      else
      {
       summaP = parseFloat($('#summa2').val());
      }
      
      if(
    (startDateInterval > endDateInterval) ||
          (endDateInterval._d == 'Invalid Date') ||
          (startDateInterval._d == 'Invalid Date')
        )
      {return false;}

    console.log('Сумма в рублях: ' + summaP);
    console.log('Сумма в рублях: ' + currentBalance);
      var timeInterval = parseInt(endDateInterval.diff(startDateInterval, 'days'));    
       console.log('Длина интервала: ' + timeInterval);

      // var dayInCurrentMonth = moment($('#targetDay').val().substr(0, $('#targetDay').val().lastIndexOf('-')), "YYYY-MM").daysInMonth();
      // console.log('Дней в месяце: ' + dayInCurrentMonth);

      // var payByDay = payByMonth / dayInCurrentMonth;
      var payByDay = tarifs[$('.form-control option:selected').val()] / 30;
      console.log('Оплата за день: ' + payByDay);    
      var totalAmount = timeInterval * payByDay - currentBalance;
      console.log('Остаток на счету: ' + currentBalance);
      // console.log('Тариф: ' +...