I have a mysql and in which i set datetime column as nvarcha

Question

I have mysql and in which i have date column set to varchar instead of date time and it store date in this format 07/17/2018 now i want o use between operator in my query but it does not work because the data type is varchar and between can only be applied on datetime column what to do now

function datevisereport()
{
global $tprefix,$con;
$data='';
$grandtotal=0;
$start_date=$_POST['start_date'];
$to_date =$_POST['end_date'];
$query="SELECT * FROM plan_voucher WHERE location=".$_SESSION['role']." and created_at between '".$start_date."' and '".$to_date."'";
$vouceher=mysqli_query($con,$query);
while($getvoucher=mysqli_fetch_array($vouceher))
{
$acamount=0;
$vdquery='select * from plan_vdetails where vid='.$getvoucher['id'].'';
$vdetails=mysqli_query($con,$vdquery);
while($getvdetails=mysqli_fetch_assoc($vdetails))
{
if($getvdetails['transtype']=='DR')
{
$amountdr=$getvdetails['amount'];
$amountdcr=0;
}
else
{
$amountdcr=$getvdetails['amount'];
$amountdr=0;
}
if($getvdetails['isrectify']==1)
{
$bgcolor='bgcolor="yellow"';
}
else
{
$bgcolor='';
}
$data.='<tr '.$bgcolor.'>
<td>'.$getvoucher['created_at'].'</td>
<td>'.$getvoucher['voucherno'].'</td>
<td width="40%">'.$getvdetails['glid'].'-'.getSingleValue('accounts','account_no',' where id='.$getvdetails['aid']).'<br>'.$getvdetails['description'].'_'.getSingleValue('employees','fullname',' where id='.$getvdetails['claim_person']).'</td>
<td>'.$amountdr.'</td>
<td>'.$amountdcr.'</td>
<td>'.$getvdetails['rbalance'].'</td>
</tr>';
$acamount=$acamount+$amountdr;
}
$grandtotal=$grandtotal+$acamount;
$data.='<tr>
<td></td>
<td></td>
<td></td>
<td style="border-bottom:1px solid balck;"><strong>'.$acamount.'.00<strong></td>
<td></td>
</tr>';
}
$data.='<tr>
<td></td>
<td></td>
<td style="border-bottom:1px solid balck;" align="right">Grand Total:</td>
<td style="border-bottom:1px solid balck;"><strong>'.$grandtotal.'.00<strong></td>
<td></td>
</tr>';
return makepdf($data,$start_date,$to_date);
}