[R] POSIXct vs Dates

Patrick Connolly p.connolly at hortresearch.co.nz
Thu Apr 22 23:43:19 CEST 2004

On Thu, 22-Apr-2004 at 07:53PM +0000, Gabor Grothendieck wrote:

|> Time zones are not part of the problem
|> yet you have to track them.  That conflicts with good design 
|> since good design means your programs don't depend on extraneous
|> elements.
|> With chron and Date there are no time zones so this type of problem
|> will not occur in the first place.

I, for one, think it's a great idea.  I'm sure it must be common to
deal only in whole days in a lot of fields.  When calculating the
difference between times, one is interested only in a precision of
days (with plus or minus 4 or 5 hours considered fair enough), yet the
difference between POSIXct times will be given as days if they're both
the same side of daylight saving changes and as seconds otherwise.
That's no big deal when used interactively, but not trivial otherwise.

I devised small functions that coerced everything to UT, but that had
to be done nearly every step of the way, otherwise the propensity to
convert back to local time kept jumping in.  It's kind of like that
software (there's plenty of it) that keeps defaulting to 'letter'
paper irrespective of how often you've set it to the proper
international standard.

Shortly after I got my GMT coercing functions working and almost ready
to make available to the list, I became aware of the forthcoming Date
class in 1.9.0, so I decided that was a better idea.

In another place and another era, it might have been termed a "Great
Leap Forward".

Great work.

Patrick Connolly
Mt Albert
New Zealand 
Ph: +64-9 815 4200 x 7188
I have the world`s largest collection of seashells. I keep it on all
the beaches of the world ... Perhaps you`ve seen it.  ---Steven Wright 

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