Here's a quick and simple digital clock you can pin to your cubicle wall. Good conversation piece and tells you when its time to go home...
For the hardware: I used a tiny2313 (you can use any tiny with at least 14 IO's), resistors, 7segment (4digits) and an RTC (DS1307) module. The module docks into a 5-pin socket so its removable for programming. I assembled them all in a proto board. I painted the board with black marker to give it a professional look ;). Power is provided by USB port (I used one from my loptop docking station)...
For the firmware: The mcu runs on 1MHz internal osc. Every second, RTC data is read via I2C (RTC time is pre-programmed separately using an arduino...). Taking the hour and minute data only, it is then separated for each 7segment digit. The digits are then cycled with 60Hz refresh rate (60Hhz x 4 or ~42ms interval between digit enable). We have interrupt on digit refresh timer, so we can spend most of the time sleeping... The seconds timer (for RTC read) is actually just a counter incremented with the digit refresh isr. The colon led is toggled every second.
With tiny 2313, you still have bytes of memory left and 3 IO's spare... With this you can still add a peizo speaker and play some funky tunes when clock hits the hour...
I'm no pro so be kind with my codes... BTW the i2c routines came from Atmel app notes which I just modified for my purpose... Pls feel free to leave a comment.