Would the STRUCTURE of the tm structure have to be changed to accommodate the solar time/lunar calendar or the oriental calendaring systems? It depends on whether you want to use "struct tm" to hold calendar dates in other systems or not. It may be possible (e.g. with a scheme like the one used to represent calculations in "terminfo" strings) to write formatting strings that will convert values from a "struct tm" into dates in various Oriental calendars. Are there weekdays in oriental? We may need to indicate the epoch somewhere (for the reign of the Emporer or whatever).
From what I saw on my trip to Japan, it seems that the only difference between Japanese and Western dates/times is that the year is given as years since the year in which the current emperor took the throne, along with the name of his reign. I.e., "now" is (modulo some transposition of fields) Sunday, 22 Mar, Showa 62 (if I remember correctly, last year was Showa 61), 6:08 AM. (Courtesy of "TZ=Japan /usr/src/bin/date" - thanks, Arthur!)
Hebrew used the fall of Adam, Is the past tense deliberate here? What calendar is currently used in Israel? However the Oriental calendar uses assension of the emperor. Or "Oriental calendars use". The Japanese calendar does so; what does the current Chinese calendar do? (I think the Soviets changed to the Gregorian calendar after the revolution; what happened post-1945 in China?) That means things change every so often. Yup. This means you can't guarantee that you can convert times in the future correctly. I'm not sure how the Japanese talk about the year 2087; "Showa 162" would sound a bit presumptuous to me. They may just punt and use the Gregorian calendar here.