Updating rhw value of a form field according to other fields.

Hi guys!
I need some help here, I'm a disaster in JS and I believe this is easy.

I need to populate the VALUE for the input field OUTBOUND and INBOUND (on the bottom) according to the value on the dropdown startDate and endDate(on the JavaScript) using format 2007-03-31.



<script>

<!-- Begin By Answering The Following Questions //;

// IF THERE IS MORE THAN ONE FORM, WHAT IS THE NAME OF YOUR FORM? //;
var FORM = '0';

// HOW MANY OPTION YEARS WOULD YOU LIKE TO DISPLAY? //;
var YEARS = '2';

// WOULD YOU LIKE TO DISPLAY PREVIOUS YEARS? YES : NO //;
var PREVIOUS = 'NO';

// WOULD YOU LIKE THE OPTION MENU'S TO LOAD AND DISPLAY THE CURRENT DATE? YES : NO //;
var DISPLAY = 'YES';

// WOULD YOU LIKE TO UPDATE THE END DATE WHENEVER THE START DATE IS CHANGED? YES : NO //;
var UPDATE = 'YES';


//###########################################################################//;
//################### #################//;
//################### NO NEED TO EDIT BEYOND THIS POINT #################//;
//################### #################//;
//################### SCRIPT AUTHOR: Willy Duitt #################//;
//################### EMAIL: WillyDuitt@hotmail.com #################//;
//################### #################//;
//################### THIS SCRIPT MAY BE FREELY USED AS #################//;
//################### LONG AS THESE CREDITS REMAIN INTACT #################//;
//################### #################//;
//###########################################################################//;

// GET TODAYS DATE //;
var today = new Date();
var thisDay = today.getDate();
var thisMonth = today.getMonth();
var thisYear = today.getYear();
if(thisYear<2000){thisYear += 1900};


// FUNCTION TO ACCOUNT FOR DAYS IN MONTHES & LEAP YEAR //;
function setMonth(theMonth,theYear){
var numDays = 31;
if(theMonth=='Apr'||theMonth=='Jun'||theMonth=='Sep'||theMonth=='Nov'){
numDays = 30;
}
if(theMonth == 'Feb' && (theYear/4) == Math.floor(theYear/4)){
numDays = 29;
}
if(theMonth == 'Feb' && (theYear/4) != Math.floor(theYear/4)){
numDays = 28;
}
return numDays;
}


// FUNCTION TO CHANGE NUMBER OF DAYS IN MONTHS //;
function setDays(select){
var form = document.forms[FORM];
var selYear = form[select+'Year'];
var selMonth = form[select+'Month'];
var selDay = form[select+'Day'];
var theYear = selYear[selYear.selectedIndex].text;
var theMonth = selMonth[selMonth.selectedIndex].text;
var numDays = setMonth(theMonth,theYear);
var theDays = selDay.length;

if(theDays > numDays){
for (var i=0; i<(theDays-numDays); i++){
selDay.options[selDay.options.length-1] = null
}
}

if(numDays > theDays){
for(var i=0; i<(numDays-theDays); i++){
newOption = new Option(selDay.options.length+1);
selDay.add(newOption);
}
}
}


// FUNCTION TO WRITE DAYS OPTION MENU //;
function writeDays(){
var optDays = '';
for(var day=1; day<=31; day++){
optDays += '<OPTION value="'+day+'">';
optDays += day+'</OPTION>';
}
return optDays;
}


// FUNCTION TO WRITE MONTHES OPTION MENU //;
function writeMonthes(){
var optMonthes = '';
var theMonthes = new Array('Jan','Feb','Mar','Apr','May','Jun',
'Jul','Aug','Sep','Oct','Nov','Dec');
for(var month=0; month<theMonthes.length; month++){
optMonthes += '<OPTION value="'+theMonthes[month]+'">';
optMonthes += theMonthes[month]+'</OPTION>';
}
return optMonthes;
}


// FUNCTION TO WRITE YEARS OPTION MENU //;
function writeYears(){
var optYears = '';
if(PREVIOUS.toLowerCase() == 'yes'){
var theYears = thisYear-parseInt(YEARS/2);
}
else{var theYears = thisYear};
for(var year=0; year<YEARS; year++){
optYears += '<OPTION value="'+(theYears+year)+'">';
optYears += (theYears+year)+'</OPTION>';
}
return optYears;
}


// FUNCTION TO WRITE START DATE MENU //;
function startDate(){
var startDate = '<SELECT style="font:Arial, Helvetica, sans-serif; font-size:11px" name="startDay" onchange="upDate(\'start\')">';
startDate+= writeDays();
startDate+= ' </SELECT>';
startDate+= '<SELECT style="font:Arial, Helvetica, sans-serif; font-size:11px" name="startMonth" onchange="upDate(\'start\')">';
startDate+= writeMonthes();
startDate+= ' </SELECT>';
startDate+= '<SELECT style="font:Arial, Helvetica, sans-serif; font-size:11px" name="startYear" onchange="upDate(\'start\')">';
startDate+= writeYears();
startDate+= ' </SELECT>';
document.write(startDate);
chkOptions('start');
}


// FUNCTION TO WRITE END DATE MENU //;
function endDate(){
var endDate = '<SELECT name="endDay" style="font:Arial, Helvetica, sans-serif; font-size:11px" onchange="upDate(\'end\')">';
endDate+= writeDays();
endDate+= ' </SELECT>';
endDate+= '<SELECT name="endMonth" style="font:Arial, Helvetica, sans-serif; font-size:11px" onchange="upDate(\'end\')">';
endDate+= writeMonthes();
endDate+= ' </SELECT>';
endDate+= '<SELECT name="endYear" style="font:Arial, Helvetica, sans-serif; font-size:11px" onchange="upDate(\'end\')">';
endDate+= writeYears();
endDate+= ' </SELECT>';
document.write(endDate);
chkOptions('end');
}


// FUNCTION TO CHECK USER DEFINED OPTIONS //;
function chkOptions(select){
var form = document.forms[FORM];
if(DISPLAY.toLowerCase() == 'yes'){
if(PREVIOUS.toLowerCase() == 'yes'){
form[select+'Year'][parseInt(YEARS/2)].selected = true;
}

else{form[select+'Year'][0].selected = true};
form[select+'Month'][thisMonth].selected = true;
form[select+'Day'][thisDay-1].selected = true;
setDays(select);
}
}


// FUNCTION TO UPDATE END DATE OPTIONS WHEN START DATE CHANGES //;
function upDate(select){
var form = document.forms[FORM];
if(UPDATE.toLowerCase() == 'yes' && select == 'start'){
form.endYear.selectedIndex = form.startYear.selectedIndex;
form.endMonth.selectedIndex = form.startMonth.selectedIndex;
form.endDay.selectedIndex = form.startDay.selectedIndex;
setDays('start');
setDays('end');
}

else{
setDays(select);
}
}


// FUNCTION TO VALIDATE FORM INPUT DATES //;
function valiDate(){
var form = document.forms[FORM];
var startDate = form.startMonth.value+' ';
startDate+= form.startDay.value+' ';
startDate+= form.startYear.value;

var endDate = form.endMonth.value+' ';
endDate+= form.endDay.value+' ';
endDate+= form.endYear.value;

if(new Date(startDate)>new Date(endDate)){
alert("Not a Valid Date!\n\nPlease choose another end date.");
return false;
}

else{ return true };
}




// End -->

</script>



<a style="font-family: Arial, Helvetica, sans-serif; font-size:12px">Depart </a><br>
<script type="text/javascript">startDate()</script>
<br>
<a style="font-family: Arial, Helvetica, sans-serif; font-size:12px">Return </a><br>
<script type="text/javascript">endDate()</script>

<input TYPE="HIDDEN" name="Outbound" value="">
<INPUT TYPE="HIDDEN" NAME="Inbound" VALUE="">

<script type="text/javascript">startDate()</script>
<br>
<a style="font-family: Arial, Helvetica, sans-serif; font-size:12px">Return </a><br>
<script type="text/javascript">endDate()</script>