mk6,
It doesn't list mine in the App Stores last 30 days either. I believe it has to do with this being an automatically pushed update by Apple. You can see the Installation in the Installations log found in System Information.
Select ( Menubar top left ) > About This Mac > On the window that pops up make sure "Overview" ( Tab ) is selected > Select "System Report..." > Select "Installations" on the left > You can see "OS X NTP Security Update" listed.
More on the update here: About OS X NTP Security Update - Apple Support
You can also verify the ntpd version, quoted below from the linked article above.
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
Hope that helps,
Weston