[R] POSIXlt vs POSIXct

MacQueen, Don macqueen1 at llnl.gov
Thu Mar 29 21:31:13 CEST 2012

I also find that POSIXct is generally the most useful, and only use
POSIXlt in special cases.

But have you considered as.POSIXct() instead of strptime()? It works for
me, and I can't remember the last time I had to use strptime() for
converting character to date/time. (But I mostly don't work with multiple
time zones, except for converting to/from UTC.


Don MacQueen

Lawrence Livermore National Laboratory
7000 East Ave., L-627
Livermore, CA 94550

On 3/29/12 12:11 PM, "Steven R Corsi" <srcorsi at usgs.gov> wrote:

>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
>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>
>>> 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
>>> 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
>>> extract needed info. Is this mostly accurate? More generally, is there
>>> resource that summarizes which date/time objects to use under which
>>> conditions? So far, I have mostly been learning by trial/error/web
>>> 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
>>>>> 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
>>>>> 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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>PLEASE do read the posting guide
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