How widespread is this time-conversion bug?

[D. J. Bernstein]

Dennis Ritchie reported on comp.lang.c.moderated that Plan 9's gmtime()
didn't believe that 2000 was a leap year. The problem is that it was
working with tm_year instead of tm_year + 1900.

Peter Seebach reported that BSD/OS had the same problem.

I wonder when these bugs were added. The tz code looks fine. I thought
BSD/OS was using tz; did they switch to another library?

---Dan