Well, many of the other date functions require KiXtart format for date, so I don't think it unreasonable to require it here also. If you want a function that is extremely forgiving with the date format, see the first version of fnDateDiff() in this thread. It uses the WbemScripting.SWbemDateTime date object and accepts virtually any valid date format and returns date using your localized short date format.

As far as the reported "bug", I'd say that it is not a bug since it is a limitation of KiX itself. If the interval between times is expected to be short, one should use "s" for the interval type.