It's assumed that Kotaro and Nobuhiko were born on September 22, 1968 - the year can be determined from the fact that they turn 19 on the day of the first episode (since the series takes place in 1987), and as for the day, it's stated that Kotaro and Nobuhiko were born on the day of a solar eclipse - and September 22 was the only day in 1968 that a solar eclipse occurred.
Troubled Production: The original head writer for Kamen Rider Black only did episodes 1-4 and 12, having seemingly left the production after that point, and there were in the end ten different writers throughout the course of the show. This explains why the show meanders so much before Shadow Moon is made the leader of Gorgom.
Working Title: Ishinomori's proposal for this series was originally titled Kamen Rider Zero-Gō ("No. 0").