Can honoring of daylight savings time be d

Doug Gwyn (ACISD/MCSB) gwyn at ARL.MIL
Tue Dec 13 04:09:19 UTC 1994

I think you (Guy) have fairly accurately covered the issue.
I agree that the POSIX rationale is confused, and wonder why they
did not seek guidance from the C standards committee since we did
thoroughly research and analyze this issue before settling on the
specification in the C standard (which intentionally contains one
or more small ambiguities to accommodate systems that cannot get
time conversion precisely right for whatever reason).  The intention
is for struct tm to contain the "user-interface" time components and
for all computations to be performed using corresponding time_t data,
which in the general C standard is a magic cookie but which is
supposed to be a uniformly ticking clock value in POSIX (with the
number of ticks per second a system macro).  Standard C provides a
difftime() function simply because some systems may not have as
clean an internal representation of time_t as UNIX does.

The oscillators is supposed to tick uniformly (with very tiny
tweaks to track an accepted time standard, but no "leaps" of any
sort), and time_t should be an affine function of the number of
accrued clock ticks.  If POSIX has managed to require that the
internal clock jump back and forth then it has a bug that should
be fixed.  "Leap seconds", like Daylight Saving Time, is an artifice
due to people having an imperfect handle on the underlying nicely
flowing physical phenomenon of time.  That's the model behind the
C standard and should be the one behind POSIX if it is to be a good

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