The DS3231 "Arduino" Board includes 4KB Ram to Save Data, you can easy compare seconds between both RTCs.. in same Time the Arduino Sketch gives Serial Output to the Rpi. I start Minicom with Fileoutput to copy the File to Public Webspace every 5 Minutes.
The Goal is to test the Internal RTC for Accuaracy smaller then 1 Minute in Month for Longtime Battery Logging (20uAh).