One form was like a shallow box. It had two wooden leaves with hollows on the inside edges, filled with wax, and space for a small wooden scriber. This permitted one to take waterproof notes in the wax without wasting money on paper. The wax could be smoothed and reused. It was probably excellent for shopping lists or other reminders.
The other form was a portable sundial. A face was on the inside of each leaf. One leaf formed a vertical sundial, the other a horizontal sundial. The shadow caster, or gnomon was a string between them, and calibrated how far open they should go (the angle is critical).
If the hinge of the diptych is level with the ground, and both dials show the same time, the dials will show the apparent solar time, the hinge faces north (in the northern hemisphere), and the gnomon is parallel with the axis of rotation of the Earth. At noon, sunset and sunrise there is no difference, but at 9am and 3pm, each degree of error in the gnomon's adjustment to latitude creates a difference of four minutes (one degree) in the two time readings. Holding a diptych at the right angle is often finicky, so many later diptychs had magnetic compasses and plumb-bobs to help.
Some diptychs had compass roses (to measure bearings to geographic features) and latitude measurement bobs. Some authorities believe that large versions (a meter or more in width) were used for maritime navigation before magnetic copasses were well-known and thereby came to acquire an air of magic in the popular mind.
Of course, all these functions could be combined in one pocket-sized artifact. Diptychs that combined writing and timekeeping often have a slot on one leaf to hold the gnomon. The gnomon can be detached from that end so the diptych can be opened completely for writing. On these the gnomon often has two knots, one for timekeeping and the other to latch the diptych shut and protect the wax. The "decorative" bead often seen on the end of extra-long gnomon cords may have been dangled as a plumb-bob to determine if the diptych's hinge was level, or to measure latitudes.
It could be a very convenient thing to keep in one's pocket.
Traditional diptychs are boxwood, with stamped hour lines and lacquered finishes. Some were also ivory (superior because it is easiest to read and less prone to wear than wood), or metal (sturdy, harder to read but less expensive than ivory).
Pocket watches are superior because they work indoors, at night and in cloudy weather.