so just before I play your date format is d/mm/yyyy or dd/mm/yyyy
i.e. 30/01/2013 if January
or 30/1o/2013 if October.
substring(now(), 0, 5) works if your date is based on now().
Also just tested if your original date field is a date control it seems to work too. Check your original date control too.
I've seen this where you set your original date field as a text field and changed it to a date field