[R] POSIXlt vs POSIXct

[Steven R Corsi]

Thanks for the link Michael. This is a very good explanation with some 
very useful tips on which date classes to use for different purposes. It 
generally strengthened the concept that POSIXct is the way to go unless 
you need to extract specific components of the date from POSIXlt. Since 
strptime() appears to be the primary conversion route from character 
class with dates/times/time zones to a date/time class, and since 
strptime() results in a POSIXlt format, that was what I was commonly 
using in past applications. That format, at times, gave me errors in 
situations where I didn't expect them. Now it is clear that the second 
step of conversion to POSIXct is preferred for many purposes.

Best Regards
Steve

===============================================
Steven R. Corsi        Phone: (608) 821-3835
Research Hydrologist   email: srcorsi at usgs.gov
U.S. Geological Survey
Wisconsin Water Science Center
8505 Research Way
Middleton, WI 53562
===============================================


On 3/29/2012 1:28 PM, R. Michael Weylandt wrote:
> That's generally my reaction to them, but you should also read "R News
> 4/1 -- Help Desk"
> (http://cran.r-project.org/doc/Rnews/Rnews_2004-1.pdf) which gives
> some tips on Date()s and the various time classes.
>
> Best, Michael
>
> On Thu, Mar 29, 2012 at 2:19 PM, Steven R Corsi<srcorsi at usgs.gov>  wrote:
>> Thanks very much for the response. That was a very good article and gives me
>> a good appreciation for the history and covers the structure of the two
>> date/time formats well.
>>
>> What I was specifically looking for is a feel for the situations when one
>> format should be used over the other. In my work, I have gotten the
>> impression that I should just use POSIXct for general useability in
>> functions and graphics until I need to extract specific date components such
>> as month, day, year, etc. In those instances, just convert to POSIXlt and
>> extract needed info. Is this mostly accurate? More generally, is there a
>> resource that summarizes which date/time objects to use under which
>> conditions? So far, I have mostly been learning by trial/error/web searching
>> which eventually is effective, but can be quite slow.
>>
>> Thanks
>> Steve
>>
>> ===============================================
>> Steven R. Corsi        Phone: (608) 821-3835
>> Research Hydrologist   email: srcorsi at usgs.gov
>> U.S. Geological Survey
>> Wisconsin Water Science Center
>> 8505 Research Way
>> Middleton, WI 53562
>> ===============================================
>>
>>
>>
>> On 3/28/2012 12:16 PM, Duncan Murdoch wrote:
>>> On 28/03/2012 10:06 AM, Steven R Corsi wrote:
>>>> Hello R users
>>>>
>>>> I am searching for a descriptive summary of the use of POSIXlt as
>>>> compared to POSIXct date/time formats. I have been using them
>>>> extensively for different purposes, but still can't quite understand
>>>> when to use which one for the most efficient coding and use. I typically
>>>> use them in graphics, comparison of times, interpolation of values
>>>> between times, computation of time-series parameters, and so on.
>>>>
>>>> My request is simply to learn if there is a resource out there that
>>>> explains the strengths of the use of each format in different situations
>>>> and if certain situations require one over the other. My web searches
>>>> have turned up basic things like the vector form (POSIXlt) vs the
>>>> decimal form (POSIXct), but I could not find specific guidance to
>>>> understand when it is best to use one over the other.
>>>
>>> The first of the "Other Topics" among the "Technical Papers" available
>>> from the main HTML help page in R should address this.
>>>
>>> Duncan Murdoch
>>
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