How widespread is this time-conversion bug?

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