Your field is likely to cause problems. It should not give December as a result when used on a document created in January.
1-1=0, not 12


The CreateDate field is linked to the creation date of the document according to Word's methods. See Using Date Fields in Microsoft Word.

For fields handling date calculations, see Paul Edstein's tutorial document. It has fields you can copy and paste into your documents. Be sure to read all the introductory material before trying to use one. The tutorial is a pinned thread in this forum. This tutorial has helped many people and the fields in it have been field tested.

Microsoft Word Date Calculation Tutorial


After reading the introductory material, look at the field on Page 7.