Hi Dmitriy thanks for the fast answer and as a reply for your question I can say:
ntpd doesn't sync time periodically (as you need to do with ntpdate) , see here
for more information (read the whole thing, it's all important. Short version: ntpd syncs the time and tells the system how fast it's drifting away from "true" time, the system then adjusts its clock frequency to properly track "true" time. Within a day most systems are tracking to within 5ms of the reference).
Absent an EXCELLENT reason you should not be mucking about with the way ntpd adjusts and disciplines your clock - these algorithms have been thoroughly tested and debugged, and given an adequate time reference will produce very small error rates. And I think that could be a great improvement in our stemcells (All of them, even those for AWS) to add ntpd in place of ntpdate.-