Leaf yellowing can have many causes, and over- and under-watering are two common possibilities. (With a drought-tolerant plant like ZZ plant, under-watering is much less likely.) A potting mix low on certain nutrients, like nitrogen, can sometimes cause premature leaf yellowing, but that is probably not the case here, especially if you're occasionally used fertilizer. Old leaves naturally turn yellow and fall off (with ZZ plant, the leaflets tend to fall off before the leaf stalk slowly shrivels), and if this is the plant's oldest current leaf, then it's reasonable and not concerning for it to be dying naturally. Sometimes plants that don't receive enough light will shed their older leaves faster than normal, because they can't make enough energy from low light levels. ZZ plant has a reputation of being very tolerant of low light, but they really do best in intermediate light levels (close to a bright window, for example).

Instead of automatically watering on a schedule (as a plant's needs change over time, and the environment, like ambient humidity and light levels, can influence how rapidly it uses water), check the potting mix instead to see how it's faring moisture-wise. To avoid overwatering, feel the potting mix at least an inch deep into the pot and only water once it's become fairly dry to the touch at that depth. If any water collects in the saucer from drainage, empty it promptly so it doesn't seep back into the drain holes and keep the potting mix too wet.

Miri