ntpd and incoming connections on OS X client?
I cannot say for certain, however, from reading this notice from NTP.ORG about a DRDos attack, I suspect someone is attempting to use your (and my) machine's NTP server for bad business.  I'm clicking the "Deny" button for accepting network connections and will go through these configuration instructions later.

Here's the opening paragraph of the notice:


  NTP users are strongly urged to take immediate action to ensure that
  their NTP daemon is not susceptible to use in a reflected
  denial-of-service (DRDoS) attack. Please see the NTP Security Notice
  for vulnerability and mitigation details, and the Network Time
  Foundation Blog for more information. (January 2014)