.ajax__calendar_title
{width:150px; margin:auto; padding:3px;}
Taken from here: http://forums.asp.net/p/1395899/3000757.aspx
If you are finding that you can’t click on the arrows on the Calendar Extender in IE 8 try using the following css.
.ajax__calendar_title{width:150px; margin:auto; padding:3px;}
It stops the calendar title from extending over the arrows.