Like the single-digit numitron clock, time is displayed by showing each of the 4 digits (as in HH:MM). But for a "clearer picture", I added the above-mentioned 4 different color LEDs.
One of its nice features is that it works on the rechargeable 3.7V LiPo battery.
The clock does not have an on-board RTC (lack of space), relying instead on the time-measuring capabilities of ATmega328 directly. So this clock will keep time for as long as the battery is kept charged (through the USB power cable).
The photo below shows my "collection" of (un-synchronized) micro clocks. (Missing is the numitron clock, which requires a stand; I am still waiting for a part to build that).
The right-most one follows the same ProMini shield idea, using an I2C OLED display and running the Pong sketch (courtesy of miker).