Hello Daniel,
Refer the below blog
C4C Analytics 101 - Designing Reports
It mentions that Key figures are nothing but measures. Characteristics are dimensions which, in lay man’s term could be just actual values as characteristic. And the numerical or amount or quantity are referred to as key figures as such.
So looking into the name of the extension field that you have created (ZChangeDate), it is not going to be shown under Key Figure since it is of type Date.
Regards,
Chandan