There was an earlier patch (first comment under http://arclanguage.org/item?id=155) that used mzscheme's date operators and avoided the platform-dependent system insanity. I advocate stamping out use of "system" from the Arc code, as it's almost guaranteed to break on different platforms. If mzscheme has taken care of platform dependence, Arc might as well take advantage of it.
As an aside, I notice that both the OpenID and "Arc at work" use system to do the heavy lifting. I propose a law that any sufficiently complicated Arc application requires use of "system" to get things done.