The document http://www.tuhs.org/Archive/PDP-11/Distributions/research/1972_stuff/Readme discusses the uncertainty regarding the epoch used for the file timestamps. "The biggest problem here is to pin down the epoch for the files. In the early version of UNIX, timestamps were in 1/60th second units. A 32-bit counter using these units overflows in 2.5 years, so the epoch had to be changed periodically, and I believe 1970, 1971, 1972 and 1973 were all epochs at one stage or another." "Given that the C compiler passes, and the library, are dated in June of the epoch year, and that Dennis has said ``1972-73 were the truly formative years in the development of the C language'', it's therefore unlikely that the epoch for the s2 tape is 1971: it is more likely to be 1972. The tape also contains several 1st Edition a.out binaries, which also makes it unlikely to be 1973." "Therefore, Warren's decoding of the s2-bits file, in s2-bits.tar.gz, uses 1972 as the epoch. However, Dennis decoding in s2.tar.gz uses 1973." "Finally, the date(1) a.out on the tape uses 1971 as its archive. How annoying! After a bit of discussion, Dennis and Warren have agreed that 1972 is the most probable epoch for s2-bits." I thought I could validate the epoch by looking at the distribution of weekdays for the three alternative years (1971 to 1973). Here are the results. wget http://www.tuhs.org/Archive/PDP-11/Distributions/research/1972_stuff/Readme for guess in 1971 1972 1973 ; do echo $guess EPOCH=$(date +'%s' -d "$guess/01/01 00:00 UTC") awk '/\/core/,/\/etc\/init/ { if ($9) print strftime("%a", '$EPOCH' + $9 / 60)}' Readme | sort | uniq -c | sort -n done 1971 1 Sat 6 Mon 8 Thu 8 Tue 17 Fri 21 Wed 34 Sun 1972 1 Sun 6 Tue 8 Fri 8 Wed 17 Sat 21 Thu 34 Mon 1973 1 Tue 6 Thu 8 Fri 8 Sun 17 Mon 21 Sat 34 Wed As you can see, unless weekends at the Bell Labs were highly atypical, 1972 has the most probable distribution of work among the days of the week.