std-unix@ut-sally.UUCP (Moderator, John Quarterman) (09/22/86)
[ I believe this is from Mark Crispin. -mod ]
From: @SUMEX-AIM.ARPA:MRC@PANDA
Date: Mon 15 Sep 86 10:32:48-PDT
Postal-Address: 1802 Hackett Ave.; Mountain View, CA 94043-4431
Phone: +1 (415) 968-1052
Has anyone taken the effort to see how other operating systems handle
this problem? If not, I'll throw one example I'm quite familiar with
into the pot.
TOPS-20 uses a 36-bit value for the time (after all it runs on a 36-bit
machine). This is a fixed point number with the decimal point located
between the halfwords expressing the number of days since midnight, 17
November 1858. Time -1 is reserved to mean "current time", therefore
the latest representable time on TOPS-20 is 7 August 2576, one second
before midnight. I guess sometime in August 2576 we'll have to fix
TOPS-20 to add another word to the time! All this is in GMT, by the
way, with no adjustments for leap seconds.
Local time is strictly a user interface consideration. The default
routines use the following cells to determine how to present the time:
TIMZON The number of hours local time lags GMT. +12 and -12 are the
same zone on different sides of the International Date Line
DSTFLG Daylight Saving Time flag:
NEVDST never use Daylight Saving Time
ALLDST always use Daylight Saving Time
0 use algorithm
DSTBGN The year in which the algorithm became effective
DSTON The last day of the year on which DST may start
DSTOFF The last day of the year on which DST may end
The present algorithm starts on the Sunday preceeding DSTON and ends
on the Sunday preceeding DSTOFF. It started in 1975, so it makes no
attempt to handle the energy conservation rules of earlier years.
The user interface will accept times in which no zone is specified
("GMT" or "PST" or "PDT", etc. will always override the TIMZON and
DST flags) and convert them into the GMT representation. Times are
output in the "current local timezone/DST" according to the rules
unless written otherwise.
This is all pretty minimal stuff. I think Unix should bite the bullet
and use at least a 48 bit time representation.
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Volume-Number: Volume 7, Number 3