This afternoon, a co-worker asked if one of the recent workbooks was the latest one. It turns out that the create date on the workbook showed that it had been created early this morning. On further examination, we saw that it actually had been created in the early afternoon. The difference between the dates happened be the time zone difference.
I am able to create a proper date on the report by calling set_properties ( create => [ gmtime() ] ) on a new workbook. I'm wondering why I need to do this... do different versions of Excel/Windows interpret the date in the workbook in different ways? Or is it possible that this is a bug?