Some of you may know about this tool already, but I just discovered Matthias Schemm’s wonderful NengoCalc, an online or offline converter of Japanese and Western dates.
Converting premodern Japanese dates to the Western calendar is extremely tricky because the years were not coterminus until Japan’s adoption of the Gregorian calendar in 1873. So, to cite a well known example, Tokugawa Ieyasu was born on the 26th day of the 12th month of the 11th year of the reign period Tembun. Most of Tembun 11 corresponds to 1542, but not all of it. As NengoCalc nicely informs us, Ieyasu was born on January 31, 1543, which happened to be a Wednesday.