[LEAPSECS] presentations from AAS Future of Time sessions

[Tom Van Baak]

> The Multics clock design (a fixed bin (71), ie double word, representing microseconds

> since 00:00 01-01-1900) clearly informs the Unix one.


Was it 1900 or 1901? See:
http://www.multicians.org/jhs-clock.html
http://web.mit.edu/multics-history/source/Multics/ldd/bos/include/rdclock.incl.alm
http://web.mit.edu/multics-history/source/Multics/doc/info_segments/datebin_.info
http://web.mit.edu/multics-history/source/Multics/doc/info_segments/date_time_.info
"If clock is not a valid date, "01/01/01 0000.0 gmt Tue" is returned."

I spent years programming on GE-635 (GCOS/TSS). Multics used the GE-645 which had a special external microsecond clock:
http://www.multicians.org/mgc.html#clock

Yes, the Multics clock is very much like the one UNIX adopted. GCOS used a more traditional date and time format: 36-bits for date (mmddyy in BCD) and 36-bits for time-of-day (in 15.625 us = 1/64 millisecond units). GCOS had a weird convention of printing all times in hours with decimal places (no minutes or seconds). That was one of the things that I liked about UNIX; it used hh:mm:ss notation. The other thing to note is "Clock readings are Multics Greenwich mean time (GMT)". UNIX also adopted this. Most OS's back then, including GCOS, ran in local time.

When I developed email in 1976 I encoded the BCD date (mmddyy) and BCD time (hhmmss) into two 18-bit binary fields. This worked because the maximum possible date was 123199, the maximum time was 235959, which just fit in the maximum half-word (2^18 = 262144).

Interesting now that I think of it, the almost 40-year old email subsystem was the only program on GCOS that could have displayed a leap second.

I want to echo what others have said on this list, that even I of all people, did not think about leap seconds when I went into the computer room or coded. Leap seconds were something exotic, for WWV, NBS and NASA. At the time, it never entered my mind that it applied to computers. All timing was based on wall clocks, telephone calls, or wrist-watches.

/tvb
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