I built one of these, modified the firmware to show leap seconds: https://www.youtube.com/watch?v=tQaPtOPP09U

I ended up making an "app" to do hundredths of seconds for TSD rallies. Awesome project. My build is 2-4 years old (IIRC), so it has some power issues, sometimes I'll catch it waking itself up more than usual, and I can hear the wakeups in the piezo which doesn't help.