One handy feature of the universe is that we can easily differentiate between elements by the number of protons within their nuclei. And for the most part, that number increases by a sensible one at a time. The result is a nice, orderly arrangement of the elements according to their proton count (their atomic numbers). And there are lots of elements, too! That opens up an interesting possibility, which is using the elements to indicate the time. Görkem Bozkurt pulled that off with his awesome Periodic Table Clock.
This clock looks like a standard periodic table drawn in black ink onto a white plastic backing. But that backing is actually translucent plastic, which allows light to shine through in a pleasing diffused way. Each element can light up independently, so you can probably see where this is going. A lit element's atomic number corresponds to the time. Blue is hours, green is minutes, and red is seconds. If, for example, seconds and minutes must share an element, like if they're both "10," then their colors will mix. It is easy to tell what colors are mixing, even if you've forgotten your color wheel, by looking at which of the three primary colors is still visible. If only one element is lit, then the time is just that number repeated (like 10:10:10).
Bozkurt's instructions call for 3D-printing the frame and topping it with a white diffuser plate, then using something like a UV printing process to lay down the black periodic table. But we would suggest filament-swapping to 3D-print the diffuser, which would let you print the black lines and text, then the white plastic over that. Doing so would eliminate the need for a complicated ink printing and would ensure correct placement of the periodic table.
The necessary elements are back lit by strips of WS2812B "NeoPixel" individually addressable RGB LEDs. An Arduino Nano board controls those LEDs and keeps time using a DS1307 RTC (real-time clock). Three push buttons give the user the ability to set the time.. The only other required parts are some fasteners and a power supply.
If you're a chemistry nerd and have your atomic numbers memorized, then this Periodic Table Clock is perfect for you.