The Arduino NTP implementation is rather naive in some respects. It basically just grabs the time in seconds from the raw packet, and then converts it to Unix time via subtraction. How would it be possible to create an implementation that is safe from at least this next NTP rollover (due in 2036)? I'd rather not design something that might suddenly fail in strange ways in a decade or two when I have forgotten why it might do so (but it's not horribly important if it's not easily fixed).
If it can make it to 2100, I'd have larger problems (the RTC would crap itself), so just this rollover is fine.
The existing implementation (https://github.com/arduino-libraries/NTPClient/blob/master/NTPClient.cpp):
unsigned long highWord = word(this->_packetBuffer, this->_packetBuffer); unsigned long lowWord = word(this->_packetBuffer, this->_packetBuffer); // combine the four bytes (two words) into a long integer // this is NTP time (seconds since Jan 1 1900): unsigned long secsSince1900 = highWord << 16 | lowWord; this->_currentEpoc = secsSince1900 - SEVENZYYEARS;