I do not have access to 2010 yet, but when running a workbook in 2003 that I had last saved in 2000, the calendar became awfully goofy. I recall either the calendar, the buttons therein, or the font (or some combination of those) becoming rather microscopic in size.
Anyways, not sure if this will help, but if you delete the calendar and plunk in a new one while in 2010, this worked for me (with 2000 to 2003). I realize that this is propbably beyond your client, but was thinking if you have access to 2010...