jQuery:
$('select').change(function() {
$('#increased').html(1 + Number($('#hours').val()) + " : " + $('#minutes').val());
});
html:
<div class="selectTime">
<select id='hours'>
<option value="12">12</option>
<option value="13">13</option>
<option value="14">14</option>
<option value="15">15</option>
</select>
:
<select id='minutes'>
<option value="15">15</option>
<option value="20">20</option>
<option value="25">25</option>
<option value="30">30</option>
</select>
</div>
<div id='increased'>
</div>