The least complex moonphase complication consists of a disk with two moons on it, with 59 teeth on its edge. The disk is indexed, or advanced, one tooth per day so
link each of the lunar disks will
link wax and wane over a 29.5-day period. This is, however, an approximation and such
link a display will be off by a full day in about two-and-a-half years.
link