I found a nice-looking "eternity calendar" in my local supermarket, it's made from metal and you're manually supposed to move around some magnets to show the current weekday and date...
As soon as I saw it, I thought it would be a great idea to stuff some LED's behind the cut out numbers in the metal to light up the weekday and date. With different colors I can even make it show the month too (green = month, red = day, for example).
Maybe somehow I can make it show the time too without modifying it too much.
Now the thing is, I want the clock to be as accurate as possible but without it costing a fortune. What I would like to do is set the time once, and not need to adjust it ever again or as seldom as possible.
What I'm thinking about is either using a 32kHz crystal or synchronize it with the frequency of the mains voltage (50Hz).
I've heard that, over time, the mains frequency is pretty much the most accurate timing source you can find here in Europe, although in periods it may drift in one direction or the other. I'm not sure if it's true, but it sounds plausible.
What would be the best way to sync my µC to the mains? 230V to 5V AC transformer, a single diode and resistor to ground, connect it to an input pin set up as digital input? Maybe with a zener to prevent over voltage on the input?