Resetting an Arduino with a timer
The following 555 circuit will produce a 170 ms low-going pulse every 24.2 hours.  You can adjust the values of the components using this calculator if you want different timing parameters.



Since the reset line of a microcontroller is configured for open-drain (meaning you can connect several inputs to it, which is a good thing since we are taking advantage of that), we need to drive it with a open-drain output.  That is what the buffer on the bottom of the circuit is for, to convert the push-pull output of the 555 to open-drain.  A typical buffer is one section of a 7407.