The challenges from the birth date form a set of four: two minor challenges from the differences among the reduced month, day and year, one main challenge derived from the difference between those two, and the whole read as a sequence unfolding with age. Because they are built by subtraction rather than addition, they model obstacle rather than gift, the counterpart to the Pinnacles, which are built by addition and read as opportunities over the same spans. Both schemes belong to the twentieth-century American numerology and rest on the conventions of modern date-reckoning.
Because it depends only on the numbers already present in a birth date, this figure is the part of the modern chart least dependent on orthography or on any particular alphabet; Walter Burkert's study of the Pythagorean tradition nonetheless finds no ancient precedent for reading obstacle by subtraction between reduced birth-date components, a method that, like the rest of the challenge apparatus, belongs to the twentieth-century American system rather than to anything older. The sense that a challenge reading names a real, recurring obstacle is also the kind of effect the psychologist Bertram Forer demonstrated in 1948, when identical, deliberately vague personality sketches handed to thirty-nine students were rated, on average, better than four out of five for personal accuracy, a result skeptics cite as the likely explanation for why a vaguely stated lifelong difficulty feels specifically true.