My system seems to be running OK now with the time retrieved at boot. 
I tried to capture the traffic from the ESP8266 using Wireshark to verify that it is not doing additional updates except for the ones I have set in these commands:
         NTP.begin ("pool.ntp.org", 1, true, 0);
         NTP.setInterval (60, 86400);   //Initial NTP call repeat 1 min, then 24h between calls
But I can see no traffic at all for the ESP8266 IP address 192.168.119.133.
My Win7 laptop is connected by wire to the same router as the ESP is connected to via WiFi, so they are on the same network (the laptop has last byte=241 and the ESP is 133.
Apparently one cannot catch packets with Wireshark in this situation, only when the ESP and PC are communication with each other...
Is there some way to verify that the ESP is only talking to NTP on startup and after that every 24 hours?