In many languages a different word for genitive is used (in English the word is the same, only prepositions "from, of" are added). If someone would like to use date/time stamp (http://www.sagehill.net/docbookxsl/Datetime.html) in both nominative and genitive versions of months names, there is no way at present.
Example: if day is specified, in Czech the genitive is used with lower case at beginning. If day is not present, the nominative is used only:
(cs) 29. srpna / Srpen
(en) August 29 / August
It would be helpful in some occasions to add new constant and into each language file to add appropriate set of genitives.