Message received, I have to check what happens if you don't get response from NTP or get an invalid response to avoid to set the RTC to a wrong time.
Looking at code, if no packet is received there is no update time. I have to add another check to the packet length, it must be 48.
In Wikipedia
I read that UDP packet has a checksum so I believe that the packet received should be correct. If packet is not correct it should be discarded and it's like not receiving any packet.
Maybe it can happen that the server is wrong or something happens. Long time ago it happened that GPS went wrong and an automatic GPS data collection didn't start because of this strange error.
If you vave frequent NTP errors it's better that you mantain the DS3231 RTC, as you can see it's only a line different.
Anyway Thingspeak uses it's time server and you can download data from it. In my opinion Thingspeak is great since you can build your own server using a PC or a little RaspberryPI so if they change idea (pachube, cosmic now Xively changed idea!) you can have your server.
Roberto