Sometimes, when specifying charges on a wand with a formula, as listed in the docs, the column formats it as a date. It happens often, but not always.
For example, on of the last items I had is a wand with 10 charges, so the Quantity is the formula "=10/50". However, the display in the "Qty" column shows Jan-00.
I'm using Excel 2010 but have seen it on Excel 2007 as well.