cfinput question - javascript, DOM, etc.

Hi, I have a form which includes the following inputs:

<cfinput type="datefield"
                           id="startDate"
                        name="StartDate"
                        value="#StartDate#"
                        message="Start Date is required!"
                        required="yes"
                        validate="usdate"
                        validateat="onsubmit">

<cfinput type="datefield"
                        id="endDate"
                        name="EndDate"
                        value="#EndDate#"
                        message="End Date is required!"
                        required="yes"
                        validate="usdate"
                        validateat="onsubmit">

I've written a JavaScript function that listens to the startDate field and if modified, sets the endDate field to the same time:

<script type="text/javascript" language="javascript">
                function populateEndDate(){                  
                    date = document.getElementById("startDate").value;
                    document.getElementById("endDate").value = date;
                }