The SNTP package contains a simple test program. Testing an SNTP client is not easy, so the test program should be considered as more a proof of concept. It shows that an NTP packet has been received, and is accurate to within a few days.
The test program starts the network interfaces using the standard call. It then starts the SNTP thread. A loop is then entered printing the current system time every second for two minutes. When the client receives an NTP packet the time will jump from 1970 to hopefully the present day. Once the two minutes have expired, two simple tests are made. If the time is still less than 5 minutes since 00:00:00 01/01/1970 the test fails. This indicates no NTP messages have been received. Check that the server is actually sending packet, using the correct port (123), correct IPv6 multicast address, and at a sufficiently frequent rate that the target has a chance to receive a message within the 2 minute interval. If all this is correct, assume the target is broken.
The second test is that the current system time is compared with the build time as reported by the CPP macro __DATE__. If the build date is in the future relative to the system time, the test fails. If the build date is more than 90 days in the past relative to the system time the test also fails. If such failures are seen, use wallclock time to verify the time printed during the test. If this seems correct check the build date for the test. This is printed at startup. If all else fails check that the computer used to build the test has the correct time.
If SNTP unicast mode is enabled, the above tests are run twice. The first time, the SNTP client is configured with NTP server addresses from DHCP. The second time, unicast mode is disabled and only multicasts are listened for. Note that the unicast test is partially bogus in the sense that any multicast packet received will also make the unicast test pass. To reduce the chance of this happening the test will wait for a shorter time for replies. This is not ideal, but it is the best that can be done with an automated test.