Comment 23 for bug 1257082

Oh, one more thing. I get the impression the failure you're experiencing, which is the case that you do have a working RTC, is a slightly different issue with the same symptom. curtin should set the RTC, and so on boot into the installed system you should already have a good enough clock. I think this would be a separate bug in curtin to set the RTC, and then this fix would only really be a workaround for you.

If I'm right about this, then using d-i rather than fast path should work in your case (I'm not sure if that's acceptable to you as a workaround or not though).