Вот вам простейший скрипт
<script type="text/javascript" src="http://localhost/script/js/jquery/jquery-1.11.2.min.js"></script>
<script type="text/javascript" src="http://localhost/script/js/MomentJS/moment-with-locales.min.js"></script>
<script type="text/javascript">
$(document).ready(function()
{
var tarifs = new Array(100, 200, 300, 400, 500, 600, 700, 800);
$('.btn-info').bind('click', evalResult);
function evalResult()
{
var temp = $('.form-control1');
temp.css({'display': 'none'});
var payByMonth = tarifs[$('.form-control option:selected').val()];
var startDateInterval = moment($('#currentDay').val());
var endDateInteval = moment($('#targetDay').val());
var timeInterval = parseInt(startDateInterval.to(endDateInteval, true).split(' ')[0]) + 1;
var dayInCurrentMonth = moment($('#targetDay').val().substr(0, $('#targetDay').val().lastIndexOf('-')), "YYYY-MM").daysInMonth();
var payByDay = payByMonth / dayInCurrentMonth;
var totalAmount = timeInterval * payByDay - parseInt($('#currentBalance').val());
$('#sum').val(totalAmount.toFixed(2));
}
});
</script>
Только поменяйте
<tr>
<td><p class="text-right form-control-static" >До какого числа желают оплатить?</p></td>
<td>
<input class="form-control" id="lastDay" type="date" name="calendar" id="targetDay">
</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 "Jump 5"</option>
<option value="5">Тариф "Jump 10"</option>
<option value="6">Тариф "Jump 35"</option>
<option value="7">Тариф "Jump 70"</option>
</select>
</td>
</tr>
Подходит?