Countdown Timer -- 2

I'm looking for someone with expertise in electronics, Arduino, microcontrollers, etc. To design a circular LCD/OLED (or similar) countdown display timer the same as this, but small and much much thinner - [login to view URL] The timer must only countdown days automatically, not hours, minutes or seconds, and be operated by a battery that you would get in a digital wristwatch or car key fob (or smaller). It needs to be simple and basic in its functionality. If I want to set the timer to countdown from 10 days then I rotate the silver outer case until I get to 10. It only requires 3 digits so the max number of days is 999. When the timer reaches 3 days the display changes colour from green to yellow and when it reaches 0 days it changes from yellow to red and flashes. When it goes past zero the next day should be -1, then -2, etc. Spec; 1. Size - it should be approx 2 inches in diameter, and as thin as possible. How thin this can be designed will decide whether to proceed or not. 2. The back would have a magnetic case so it can attach to metal surfaces.