[SalesForce] Date picker on Force.com..

Are you trying to get the value from the drop-down, or set the value? Because, from what I see from your code, you are using .val(value) which is a setter method, rather than .val() which is a getter method.

I.e.

// Gets the value from the selected option in the <select name="title"> element
var titleField = $dialogContent.find("select[name='title']").val();


// Sets the selected option in the <select name="title"> element
var doctorId = '00Ab00000057v4B';
$dialogContent.find("select[name='title']").val(doctorId);

Keep in mind, this could be timezone differences. Most likely when you are logged in, created date is converted to your current user's timezone, while in sites it's most likely GMT. Please add your timezone to your create date format and you'll be able to determine if this is the case.

<apex:outputText value="string: {0,date,yyyy.MM.dd 'at' HH:mm:ss z}"> 
    <apex:param value="{!account.CreatedDate}" /> 
</apex:outputText>

Link to your simple date format doc