Cfinput question - javascript, DOM, etc.

Question

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;

Answer 1

Thanks Dan,
I tried onBlur already. The problem seems to be that when you use the date popup, although the input field gets populated with a valid date, it doesn't behave like you actually clicked in the field, or edited it, or left it. So if you use onBlur and enter a date using the popup, you need to also then click in the input field and then click elsewhere for onBlur to trigger, which is obviously not what I want.
So I figured that a better way to do it would be to attach an eventListener to the input field, which would theoretically be able to register any change that occurred in that field, whether it was from direct input or via the Date popup. But I'm stuck because I can't figure out exactly how to do this.