is this a JS problem

Hi guys, my programming is a pasttime but still important to me. The following
code is a payment doc. allowing partial payments. Balances are put in "prevbal"
field. Complete payments get a "P" code. When the paidamt is keyed in, the
current date is autoinserted. If a recurring debt the amtdue stays the same.
If it is, the amtdue is calculated. I'm unsure if the failure is in the
javascript or not?

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The javascript follows:

function $_(IDS) { return document.getElementById(IDS); }
function calculate_paid()
  {
  var status = document.getElementById("status");
  var pd = document.getElementById("pd");
  var payec = document.getElementById("payec"); 
  var paidamt = document.getElementById("paidamt");
  var amtdue = document.getElementById("amtdue");
  var prevbal = document.getElementById("prevbal");
  var shipamt = document.getElementById("shipamt");
var datepaid = document.getElementById("datepaid");

  var dateNow = new Date();
  var dayNow = dateNow.getDate();
  var datePaid = (dateNow.getMonth()+1)+"-"+dateNow.getDate()+"-"+dateNow.getFullYear();
datepaid.value = datePaid;
var owed.value == parseFloat(amtdue.value) + parseFloat(shipamt.value) +  parseFloat(prevbal.value) - parseFloat(paidamt.value);
if(status=="N"){amtdue.value = owed.value;}
if(owed=="0.00") { pd.value = "P"; shipamt="0.00", prevbal="0.00";}
  }

Code (markup):

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<td><input type='text' readonly size=15 name='acctno' value='" . $row['acctno'] . "' ></td>
<td><input type='text' readonly size=25 name='bname' value='" . $row['bname'] . "'></td>
<td><input type='text' readonly size=25 name='purpose' value='" . $row['purpose'] . "'></td>
<td><input type='text' size=7 id='paidamt' name='paidamt' value='" . $row['paidamt'] ."'
  onblur='calculate_paid(this)'></td>
<td><input type='text' size=7 id='amtdue' name='amtdue' value='" . $row['amtdue'] . "' ></td>
<td><input type='text' size=7 id='prevbal' name='amtdue' value='" . $row['prevbal'] . "' ></td>
<td><input type='text' size=10 id='datepaid' name='datepaid' value='" . $row['datepaid'] . "' ></td>
<td><input type='text' size=3 id='pd' name='pd' value='" . $row['pd'] . "' ></td><br />
<td><input type='submit' name='update' value='Make Payment' />
</tr>";
  }
echo "</table>
<input type='submit' name='update' value='Update Record' />
</form>";
  }
else{echo "invalid entry<br />Select another?<br />";}
  }
if(!empty($_POST["update"]))
  {
  $sql = "UPDATE oocust SET 
  amtdue = '" . mysql_real_escape_string($_POST['amtdue']) . "',
  paidamt = '" . mysql_real_escape_string($_POST['paidamt']) . "',
datepaid = '" . mysql_real_escape_string($_POST['datepaid']) . "',
  pd = '" . mysql_real_escape_string($_POST['pd']) . "'
  WHERE acctno='".$_POST['acctno']."'";
mysql_query($sql) or die(mysql_error());
echo "Success!";
  }
  ?>
<form method="post" action="#"><br />
<input type="text" name="acctno"/> <p>
<input type="submit" name="submit" value="Select"/>
</form></body></html>

Code (markup):