this topic comes up from time to time. the extra realtime component is a save bet if you use it correctly. still an arduino should be enough to build an accurate clock. if you notice an offset (because of crystal tolerances) you should also be able to compensate for it in your code. as kg4wsv wrote, the actual frequency is dependant on the crystal but shouldn't change over time. me too says go ahead! and keep us informed. it's a shame that there's no arduino-only clock code out there yet.
ku