So, I looked at the code, and the bug is not obvious. (The routine for detecting leap years in V6's ctime.c is so minimalistic it actually works for 2000!) The version I'm currently running had been worked on some, to have a hack fix for the overflow of the number of 8-hour periods since the epoch ("hack" since the 'fixed' version will break in 2029 or so), and I was worried that 'fix' was wrong and caused the week-day problem, but I don't think so. Noel