Fifty years ago today, #Unix was complete and useful enough that it got its first ever manpages. Unix First Edition was born: https://www.bell-labs.com/usr/dmr/www/1stEdman.html
But #Unix First Edition was complete enough that it began to be used for paperwork production, using ed(1) and roff(1). I can only assume that this is how the manual was written.
(Today, of course, paperwork production in *nix is done using factions of the endless Editor Wars and typesetting systems like groff(1), TeX/LaTeX, or conversion to It Looks Like You're Writing A Letter.)
Also by the time of First Edition, #Unix was running on a PDP-11, not the PDP-7 it had been originally designed and built on. The team would use the pain of porting an entire OS using only Assembly language to invent the slightly more portable C language the following year.
At the time, #Unix time was not the Unix time we know today. It was 60ths of a second since 1971-01-01, just 307 days prior, and which would roll over 521 days later, about 1973-04-08.
The epoch had been reset at least once before, given its origin in 1969, and it would be reset and still use 60ths at least once more before it was finally reset to 1970 and standardized to the whole-seconds we know today.