'Event edit' (/eventedit) uses name='t' to identify the 'time' field in form. 
'Edit Report Format' (/rptedit?rn=10) uses name='t' to identify the 'title' field in form. 

The simplest was to resolve this is to change t to time or title respectively

Note: 
I added the following to the header to allow the use of a date picker for the 'Event edit' and the new and edit ticket screens. :

<pre>
<link rel="stylesheet" href="$baseurl/doc/tip/jquery/jquery-ui-1.8.9.custom.css" type="text/css" media="screen">

<script src="$baseurl/doc/tip/jquery/jquery-1.4.4.min.js" type="text/javascript"></script>
<script src="$baseurl/doc/tip/jquery/jquery-ui-1.8.9.custom.min.js" type="text/javascript"></script>

<script type="text/javascript"> 
// always use datepicker for this field
 $(function() {
  $( "input[name='t']" ).datepicker({
			numberOfMonths: 2,
			showButtonPanel: true,
			dateFormat: 'yy-mm-dd'
		});
  });
</script>

<script type="text/javascript">  
    var datefield=document.createElement("input")  
    datefield.setAttribute("type", "date")  
if (datefield.type!="date"){ //if browser doesn't support input type="date", initialize date picker widget:  
    jQuery(function($){ //on document.ready  
 $( "input[name='last_useful_date']" ).datepicker({dateFormat: 'yy-mm-dd'});  
 $( "input[name='date_closed']" ).datepicker({dateFormat: 'yy-mm-dd'});  
    })  
}  
</script> 
</pre>

form fields sharing names for different purposes make it difficult to use javalscript library