A GPS module can easily provide accurate sub-second time, and with a little more effort, much greater accuracy. Also shows latitude/longitude, altitude, speed, visible satellites.
Technical details: UBLOX NEO-6 GPS module, Arduino Nano, nRF24L01+ radio transceiver in the GPS clock, which has both LCD (I2C) and 7-segment MAX7219 driver) LED displays. A flashing decimal point shows each NMEA sentence parsed and the rightmost digit indicates a valid, recent fix. A single 8mm Neopixel (not shown here) indicates the age of the fix, fading from green to red over time until satellite fix is re-established. And it also tell the time.