You have to modify the date_zimlet (or create a new one).
For us French, Carlos created a new zimlet (date_zimlet_fr) with the correct regular expression in it.
You can download from the forum (
here) the french patch (that contains the french date zimlet, a patch for aspell and a couple other things).
You'll then be able to compare the french date zimlet with the original one to find out what/where to change the regexp for your needs.