Reset pin and power-on reset are two different cases. Which one do you really mean?
Power-on reset without some battery backup is not really possible. Unless you don't care for the date time staying frozen at the time of power down.
Reset pin: Just make sure you don't clear the variables holding the date time during initialization. Possibly add some CRC to verify correctness. Again, the date time will not remain too accurate over repeated resets out very long reset durations.