FW: Questions

[Michel Bourget]

On Wed, 2007-04-04 at 18:00 -0400, Fan_Zhenqiang at emc.com wrote:
>  Hi Michael,
> Thank you for your reply. 
> Sorry that I used ambiguous example. What I really asked is:
> 
> Is there a function that takes a time zone, say America/Los_Angles, and
> a date value such as "2007-11-04 01:30", and tells whether the given
> date/time value is a DST in that time zone? If no, how easy is it to
> write such a function?
> 

No, the function is not available.

Given a known format , you can convert any ascii date-time-whatnot 
to a 'struct tm' data with strptime(); then use localtime() recipe given
previously. Also account for TZ and and print out tm_isdst. It looks
easy.
 
Good luck !

> Thanks again.
> 
> Zhenqiang Fan
> 
> -----Original Message-----
> From: Michel Bourget [mailto:michel at sgi.com] 
> Sent: Wednesday, April 04, 2007 12:39 PM
> To: tz at elsie.nci.nih.gov
> Cc: Fan, Zhenqiang; tz at lecserver.nci.nih.gov
> Subject: Re: FW: Questions
> 
> On Wed, 2007-04-04 at 10:41 -0700, Paul Eggert wrote:
> > Fan_Zhenqiang at emc.com writes:
> > 
> > > What if a time zone is specified together with the date/time value?
> For
> > > instance, is there any function that call whether "2007-11-04 01:30
> PST"
> > > is DST?
> > 
> > Sorry, no; in general that's not possible, as in some locations the
> > same abbreviation is used for both standard and daylight time.  One
> > could do a reasonable job in most cases, as in the other problems you
> > mentioned; it shouldn't be that hard, but the functions aren't there
> > now.
> 
> Bottom line, you need to know the *context*( the geographic location )
> of the date, hence the Country/City TZ value information, and not the
> abbreviation ( DST, PST, WAT, ... ). Then, as explained earlier, use
> localtime() returned 'struct tm'->tm_isdst value. Unfortunately, the %Z
> format still gives the abbreviation, not the time zone itself. 
> 
> Briefly. To compute isdst, you need date and TZ( not its abbreviation ).
> 
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