The following 555 circuit will produce a 1.4 second low-going pulse every 23.9 hours. You can adjust the values of the components using this calculator if you want different timing parameters.
Note the value of C -- that's 0.1 F, not 0.1 µF. This large value is needed to get the long time period of 24 hours.
Since the reset line of a microcontroller 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.