If you want the most accurate DIY clock possible have a look at this DIY Atomic Clock Synced LED Clock build. This clock is built to use the DCF77 which stands for D=Deutschland (Germany), C=long wave signal, F=Frankfurt, 77=frequency: 77.5 kHz.

Via: LED Projects

“The 77.5 kHz carrier signal is generated from local atomic clocks that are linked with the German master clocks in Braunschweig. With a relatively high power of 50 kW, the station can be received in large parts of Europe, as far as 2000 km from Frankfurt (and further away depending on signal propagation and local interference). As an example, reception with high-grade consumer clocks is possible in Portugal and Gibraltar (during night hours). Its signal carries an amplitude-modulated, pulse-width coded 1 bit/s data signal. The same data signal is also phase modulated onto the carrier using a 512-bit long pseudorandom sequence (direct-sequence spread spectrum modulation). The transmitted data repeats each minute.”