How many NTP implementations don't consider era or contain bugs if era != 0?
To fix this you need a persistent low water mark on the time, compiled into the NTP program and/or stored in the filesystem (eg the timestamp on the NTP drift file). Then NTP timestamps can be interpreted as spanning the 136 years after the low water mark, using modular arithmetic.
Yep, this is what many GPS receivers do to deal with the 20 year issue. They burn their manufacturing date, so they know which "cycle" they're in.
> The NTP protocol will synchronize correctly, regardless of era, as long as the system clock is set initially within 68 years (a half-era) of the correct time