+++ This bug was initially created as a clone of Bug #734993 +++
[skip to the relevant part]
--- Additional comment from nphilipp on 2011-09-09 11:28:05 EDT ---

Meanwhile I could reproduce the problem and it's two-fold, the triggering bug is in s-c-date, but a followup one is in firstboot, specifically in its date module:

[...]
    def apply(self, interface, testing=False):
        if testing:
            return RESULT_SUCCESS

        try:
            rc = self.scd.firstboot_apply()
            if rc == 0 and self.scd.closeParent:
                return RESULT_SUCCESS
            else:
                return RESULT_FAILURE
        except:
            return RESULT_FAILURE
[...]

--> this code catches all exceptions raised from self.scd.firstboot_apply() (which makes debugging hard) and apparently (I'm guessing here) causes firstboot to not advance any further in this step (because of "return RESULT_FAILURE"?).
[...]

I think firstboot should ask the user if they want to skip that step if it catches exceptions and alternatively/additionally offer to report them to e.g. Bugzilla.