How to get child page parameters by iframe parent page

1. The iframe in the parent page

The code copy is as follows:

<iframe name="parentPage"></iframe>

2. Properties of elements in subpage

The code copy is as follows:

<input type="text" id="date" data-dojo-type="dijit/form/DateTextBox" value="2013-11-12"/>

3. Take the median value of DateTextBox in the subpage

The code copy is as follows:

var statisticsDate = window.frames["parentPage"].document.getElementById('date').value;