[R] POSIXlt vs POSIXct

Steven R Corsi srcorsi at usgs.gov
Thu Mar 29 20:19:40 CEST 2012

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.


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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