Concatenar campos do Form no Insert

Olá galera preciso de uma ajuda com um insert que to precisando fazer

seguinte:

tenho um campo de data dividido em 3 select input
preciso juntar eles e gravar em um unico campo do banco de dados

segue abaixo o insert:

if ((isset($_POST["MM_insert"])) && ($_POST["MM_insert"] == "form1")) {
 $insertSQL = sprintf("INSERT INTO tbprojeto (pjtnumero, pjtcliente, pjtdia, pjtevento, pjtlocal, pjtcidadeuf, pjtitens, pjtdescricao, pjtfrete, pjtstatus) VALUES (%s, %s, %s, %s, %s, %s, %s, %s, %s, %s)",
 GetSQLValueString($_POST['pjtnumero'], "text"),
 GetSQLValueString($_POST['pjtcliente'], "text"),
 GetSQLValueString($_POST['y'] . "-" . $_POST['m'] . "-" . $_POST['d'], "text"),
 GetSQLValueString($_POST['pjtevento'], "text"),
 GetSQLValueString($_POST['pjtlocal'], "text"),
 GetSQLValueString($_POST['pjtcidadeuf'], "text"),
 GetSQLValueString($_POST['pjtitens'], "text"),
 GetSQLValueString($_POST['pjtdescricao'], "text"),
 GetSQLValueString($_POST['pjtfrete'], "text"),
 GetSQLValueString($_POST['pjtstatus'], "text"));

Segue os campos do formulario:

 <select id="d" class="styledselect-day">
 <option value="">dd</option>
 <option value="1">1</option>
 <option value="2">2</option>
 <option value="3">3</option>
 <option value="4">4</option>
 <option value="5">5</option>
 <option value="6">6</option>
 <option value="7">7</option>
 <option value="8">8</option>
 <option value="9">9</option>
 <option value="10">10</option>
 <option value="11">11</option>
 <option value="12">12</option>
 <option value="13">13</option>
 <option value="14">14</option>
 <option value="15">15</option>
 <option value="16">16</option>
 <option value="17">17</option>
 <option value="18">18</option>
 <option value="19">19</option>
 <option value="20">20</option>
 <option value="21">21</option>
 <option value="22">22</option>
 <option value="23">23</option>
 <option value="24">24</option>
 <option value="25">25</option>
 <option value="26">26</option>
 <option value="27">27</option>
 <option value="28">28</option>
 <option value="29">29</option>
 <option value="30">30</option>
 <option value="31">31</option>
 </select>
 </td>
 <td>
 <select id="m" class="styledselect-month">
 <option value="">mmm</option>
 <option value="1">Jan</option>
 <option value="2">Feb</option>
 <option value="3">Mar</option>
 <option value="4">Apr</option>
 <option value="5">May</option>
 <option value="6">Jun</option>
 <option value="7">Jul</option>
 <option value="8">Aug</option>
 <option value="9">Sep</option>
 <option value="10">Oct</option>
 <option value="11">Nov</option>
 <option value="12">Dec</option>
 </select>
 </td>
 <td>
 <select id="y" class="styledselect-year">
 <option value="">yyyy</option>
 <option value="2008">2008</option>
 <option value="2009">2009</option>
 <option value="2010">2010</option>
 <option value="2011">2011</option>
 <option value="2012">2012</option>
 <option value="2013">2013</option>
 <option value="2014">2014</option>
 <option value="2015">2015</option>
 <option value="2016">2016</option>
 <option value="2017">2017</option>
 <option value="2018">2018</option>
 <option value="2019">2019</option>
 </select>