[TUHS] Happy birthday, Unix timestamp!

[Norman Wilson]

Andrew Hume:

  if i recall correctly, V1 of Unix had time measured in milliseconds.
  were folks that sure that this would change before wrap-around?

====

Not milliseconds (which were infinitesimally small to the
computers of 1969!) but clock ticks, 60 per second.

Initially such times were stored in a pair of 18-bit PDP-7
words, giving a lifetime of about 36 years, so not so bad.

The PDP-11's 16-bit words made that a 32-bit representation,
or about two and a quarter years before overflow.  Which
explains why the time base was updated a few times in early
days, then the representation changed to whole seconds, which
in 32 bits would last about as long as 36 bits of 60 Hz ticks.

The PDP-7 convention is documented only in the source code,
so far as I know.  The evolution of time on the PDP-11 can
be tracked in time(II) in old manuals; the whole-seconds
representation first appears in the Fourth Edition.

Norman Wilson
Toronto ON
Not that old a timer, but once looked into old time