MarkT:
Many current Arduino boards do not keep accurate time as they use ceramic
resonators, which are accurate typically to +/-0.3% (several minutes a day).A quartz crystal can be substituted on some of the boards if you have the
soldering skills, giving more like +/-50ppm (a few seconds a day accuracy).
so how do you know if your Arduino has a ceramic or crystal resonator? Cheching the schematic for the uno, the resonator is just described as "XTAL". Can you tell by the physical appearance, eg does a metal can mean a crystal?