Issue DECODE-UNIVERSAL-TIME-DAYLIGHT Writeup

Issue:        DECODE-UNIVERSAL-TIME-DAYLIGHT

References:   DECODE-UNIVERSAL-TIME (p445)

Category:     CLARIFICATION

Edit history: 20-Jun-88, Version 1 by Pitman

		30-Sep-88, Version 2 by Masinter


Problem Description:


  The description of DECODE-UNIVERSAL-TIME does not say what happens with

  TIME-ZONE. Since the description of ENCODE-UNIVERSAL-TIME mentions that

  its behavior differs depending on whether a time zone is explicitly passed,

  some implementors may have assumed that DECODE-UNIVERSAL-TIME should do

  likewise.


  Even if all implementations did the same thing, it should still be clarified

  whether an implementation were permitted to return dsp=NIL tz=n rather than

  dsp=T tz=n+1 for time zones in which daylight savings time was believed to

  be (or known to be) in effect. Currently, you cannot tell whether "NIL" for

  daylight-savings-time-p means "daylight savings time is in effect" or

  just "daylight savings time is not known to not be in effect".


  These tools appear to be more portable than they are.


Proposal (DECODE-UNIVERSAL-TIME-DAYLIGHT:LIKE-ENCODE):


  Specify that, like ENCODE-UNIVERSAL-TIME, DECODE-UNIVERSAL-TIME ignores

  daylight savings information if a timezone is explicitly specified.


Rationale:


  This makes things consistent with ENCODE-UNIVERSAL-TIME.


Test Case:


  ;; ### This test case relies on time zone not changing in real

  ;; ###   time, in defiance of warning in note at bottom

  ;; ###   of p445.


  (LET* ((HERE (NTH 8 (MULTIPLE-VALUE-LIST (GET-DECODED-TIME)))) ;Time zone

	 (RECENTLY (GET-UNIVERSAL-TIME))

	 (A (NTHCDR 7 (MULTIPLE-VALUE-LIST (DECODE-UNIVERSAL-TIME RECENTLY))))

	 (B (NTHCDR 7 (MULTIPLE-VALUE-LIST (DECODE-UNIVERSAL-TIME RECENTLY HERE)))))

    (LIST A B (EQUAL A B)))


Under this proposal, this would return ((T 5) (NIL 5) NIL) in EDT, for example. 


Current Practice:


 Symbolics Genera, Symbolics Cloe, Lucid 3.0 and Envos Medley seem to 

 implement this proposal. Some other implementations do not.


Cost to Implementors:


  The cost of changing this should be trivial.


Cost to Users:


  This feature is already not well-defined since no portable program can rely

  on the current behavior, so the cost is small.


Cost of Non-Adoption:


  The time primitives are considerably less useful if this point is not

  clearly spelled out.


Benefits:


  The cost of non-adoption would be avoided.


Aesthetics:


  Anything that improves the intelligibility of language primitives improves

  language aesthetics.


Discussion:


  An alternative would be to specify that, unlike ENCODE-UNIVERSAL-TIME,

  DECODE-UNIVERSAL-TIME treats daylight savings information the same

  regardless of whether a time zone argument is explicitly or not. This seems 

  actually to be what was intended originally.


  This problem arose while trying to port Macsyma between different

  Common Lisp implementations. The cleanup committee does not have

  a strong opinion on this matter, as long as the behavior is specified.