[R] Why is there error in as.POSIXlt.character when using strftime()?

[Jeff Newmiller]

If there are no time-of-day values in your data set the the Date type is great. However, it can be messy if you work with both Date and POSIXt types in the same analysis... I recommend sticking with one or the other.

The trunc.POSIXt function is more appropriate for getting POSIXt dates than converting to character and back.

On February 2, 2019 9:03:31 AM PST, C W <tmrsg11 using gmail.com> wrote:
>Also, I was able to extract date using
>> as.Date(dat, "%m/%d/%y")
> [1] "2015-07-12" "2015-07-12" "2015-07-12" "2015-07-12" "2015-07-12"
> [6] "2015-07-12" "2015-07-12" "2015-07-12" "2015-07-12" "2015-07-12"
>[11] "2015-07-12" "2015-07-12" "2015-07-12" "2015-07-12" "2015-07-12"
>[16] "2015-07-12" "2015-07-12" "2015-07-12" "2015-07-12" "2015-07-13"
>[21] "2015-07-13" "2015-07-13" "2015-07-13" "2015-07-13" "2015-07-13"
>[26] "2015-07-13" "2015-07-13" "2015-07-13" "2015-07-13" "2015-07-13"
>[31] "2015-07-13" "2015-07-13" "2015-07-13" "2015-07-13" "2015-07-13"
>[36] "2015-07-13" "2015-07-13" "2015-07-13" "2015-07-13" "2015-07-13"
>
>> class(as.Date(dat, "%m/%d/%y"))
>[1] "Date"
>> class(strftime(strptime(dat, format="%m/%d/%y %H:%M"), format =
>"%m/%d/%y"))
>[1] "character"
>
>If I want to date caluculation, would as.Date() be more desirable? Or
>does
>strftime() do the same thing?
>
>
>
>On Sat, Feb 2, 2019 at 11:51 AM C W <tmrsg11 using gmail.com> wrote:
>
>> Thank you all very much, very helpful! :)
>>
>> I'm just curious, what does strptime(), strftime(), as.POSIXlt.(),
>and
>> as.POSIXct() stand for?
>>
>> On Sat, Feb 2, 2019 at 10:41 AM Jeff Newmiller
><jdnewmil using dcn.davis.ca.us>
>> wrote:
>>
>>> ... and in general, you need to specify the time zone to avoid
>surprises.
>>> In many cases this can be as simple as
>>>
>>> Sys.setenv(TZ="GMT")
>>>
>>> but it can be specific to your data set also.
>>>
>>> On February 2, 2019 7:09:46 AM PST, Duncan Murdoch <
>>> murdoch.duncan using gmail.com> wrote:
>>> >On 01/02/2019 10:45 p.m., C W wrote:
>>> >> Dear R community,
>>> >>
>>> >> I am working with dates. And I get the following error:
>>> >>> strftime(dat[20], format="%H:%M")
>>> >> Error in as.POSIXlt.character(as.character(x), ...) :
>>> >>    character string is not in a standard unambiguous format
>>> >
>>> >You are using the wrong function:  strftime() formats a time object
>as
>>> >a
>>> >character string.  You want strptime() to convert character (or
>factor
>>> >in your case) to a time object.
>>> >
>>> >But you need to give the format for the full string, not just the
>time
>>> >at the end.
>>> >
>>> >If you really were intending to extract times from dat, then you
>need
>>> >both conversions:
>>> >
>>> > > strftime(strptime(dat, format="%m/%d/%y %H:%M"), format =
>"%H:%M")
>>> >  [1] "11:32" "11:42" "12:17" "12:31" "12:50" "14:10" "14:19"
>"14:59"
>>> >"15:57" "16:00" "16:46" "16:51" "17:35" "17:59" "18:17" "19:07"
>"19:08"
>>> >[18] "19:31" "21:21" "06:00" "06:20" "06:37" "06:40" "06:46"
>"07:20"
>>> >"07:47" "07:50" "07:54" "08:11" "08:23" "08:31" "08:33" "08:43"
>"09:04"
>>> >[35] "09:09" "09:30" "09:59" "10:01" "10:03" "10:05"
>>> >
>>> >Duncan Murdoch
>>> >
>>> >______________________________________________
>>> >R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
>>> >https://stat.ethz.ch/mailman/listinfo/r-help
>>> >PLEASE do read the posting guide
>>> >http://www.R-project.org/posting-guide.html
>>> >and provide commented, minimal, self-contained, reproducible code.
>>>
>>> --
>>> Sent from my phone. Please excuse my brevity.
>>>
>>

-- 
Sent from my phone. Please excuse my brevity.