6 Month Countdown Timer Circuit
Based on this answer, I think a 555 won't work. You'd need a massive capacitor and a huge resistor, and the accuracy would be terrible.

I'd recommend using the simplest of 8-bit microcontrollers. You'd have to handle the numbers carefully, but a PIC16F84A (or an Arduino, or lots of other similar devices) could do it. I might set up a 1 second timer, and then count 15778463 timer expirations, storing the count in a 32-bit variable. (For Arduino 32-bit variables, see http://www.arduino.cc/en/Reference/Double)