Apple fixed a critical security issue with an update. Its not an automatic and seamless update as said. You will have to enable security updates to have a automatic download and update.
OS X NTP Security Update
Available for: OS X Mountain Lion v10.8.5, OS X Mavericks v10.9.5, OS X Yosemite v10.10.1
Impact: A remote attacker may be able to execute arbitrary code
Description: Several issues existed in ntpd that would have allowed an attacker to trigger buffer overflows. These issues were addressed through improved error checking.
To verify the ntpd version, type the following command in Terminal: what /usr/sbin/ntpd. This update includes the following versions:
- Mountain Lion: ntp-77.1.1
- Mavericks: ntp-88.1.1
- Yosemite: ntp-92.5.1