I have a project that will be running from DC power and needs a very accurate real time clock. The device is basically a transmitter that will transmit the time to a receiver. The receiver will compare the received time to the actual time as indicated by SNTP servers. The time on the transmitter must be accurate to approximately 1 minute per year with a temperature specification of -40°C to +85°C.
The best time keeper I think of would be GPS based, but I'm not sure I can guarantee that a GPS signal be available so GPS might not work. The second best time keeper I can think of are the NIST time radio stations. I don't think those are standardized world wide, so a single solution might not be doable. The best RTC I can find is the DS3231 / DS3234 with a 3.5ppm, internal, temperature compensated crystal on the -40 to +85°C range. This comes out to about 2 minutes per year, or half of the desired accuracy we're looking to achieve.
I've seen key fobs that display a number on an LCD screen that changes every few minutes. How do those keep accurate time? Or does the server just allow a significant amount of "slop" in the time?