To accomplish the process of getting datebox content which is placed inside Iframe you can follow this:
You need to write function as below in your iframe document which returns datebox value.
Code
function getValue(value){
var id='#'+value;
var val= $(id).datebox('getValue');
return val;
}
From parent document you can call getValue() method and pass datebox component ID as argument to the getValue() method.
Code to call getValue() from Iframe parent document
value = window.frames[0].getValue('field ID');
alert(value);
Thank you