Consider this: the internal oscillator of a micro is said to have an accuracy of +/-10% over the nominal frequency clock... External resonators have similar problems.
Using the swRTC on an Arduino UNO with a resonator will result in differences that can reach the value of minutes/day. But you can use the deltaT to try to adjust them. I think that with a 2009, that has an external crystal for the Atmega, you will reach more precision.
This lib is intended to be useful for people that need to track the time without the necessity to have a big accuracy.