[R] Reading a datetime vector

Jeff Newmiller jdnewmil at dcn.davis.ca.us
Mon Feb 22 18:30:58 CET 2016

It is not minutes... read the Excel documentation for representing dates... it is days since December 30, 1899 on Windows.  Read the links I provided in my last email. 

Also read ?str ... that function does not return anything... it only prints out information so don't expect to get anything useful by assigning the output of that function to a variable. 

Also read the examples section of the help file ?read.xlsx2 for relevant help. 
Sent from my phone. Please excuse my brevity.

On February 22, 2016 8:55:34 AM PST, D Wolf <doug45290 at yahoo.com> wrote:
>Hello Everyone, 
>The column begins populated with integers as so:1/1/2013 0:00 in the
>spreadsheet equals 41257 in R's dataframe1/1/2013 0:15 in the
>spreadsheet equals 41257.010416666664 in R's dataframe...41257 must be
>in minutes since 1440min/day * .010416666664 day = 15 minutes. 41257
>minutes is about 29 days: 41257 min / 1440 min/day = 28.65 days. So I
>don't know why the dataframe is showing 41257 for 1/12013 0:00. 
>Oddly, R sees the vector as NULL despite the fact it has integers in
>each record in the column:data_type = str(df2_TZ$DateTimeStamp)
>produces a NULL (empty) variable. 
>I tried:
>df2_TZ = read.xlsx2("DF_exp.xlsx", sheetName = "Sheet1")Sys.setenv(TZ =
>"GMT")testdtm <- as.POSIXct(df2_TZ$DateTimeStamp, format = "%m/%d/%Y
>%H:%M")# Inspect the resulttestdtmstr(testdtm)
>testdtm is a vector filled with NA values, which figures since
>DateTimeStamp is NULL. 
>I noticed in the table on page 32 of the R Help Desk pdf you linked to
>that dp-as.POSIXct(format(dp, tz="GMT")) is the only option listed for
>time zone difference. So I tried:df2_TZ = read.xlsx2("DF_exp.xlsx",
>sheetName = "Sheet1")df2_TZ_seq <- as.POSIXct(format(dt2_TZ, tz="GMT"))
>and got: Error in format(dt2_TZ, tz = "GMT") : object 'dt2_TZ' not
>Is the vector neither character nor factor, since it's NULL? Where do I
>go from here? 
> Thank You,Doug
>Hi Doug,What you have done is to ask whether the character string
>"DF_exp.xlsx" is a character string. I think Yogi Berra, were he still
>around, could have told you that. What will give you some useful
>information is:
>which asks for information about the object, not its name.
>On Friday, February 19, 2016 12:41 PM, Jeff Newmiller
><jdnewmil at dcn.davis.ca.us> wrote:
>This is a mailing list. I don't know how you are interacting with it...
>using a website rather than an email program can lead to some confusion
>since there can be many ways to accomplish the task of interacting with
>the mailing list. My email program has a "reply-all" button when I am
>looking at an email. It also has an option to write the email in plain
>text, which often prevents the message from getting corrupted
>(recipient not seeing what you sent to the list).
>Using the str function on a literal string (the name of a file) will
>indeed tell you that you gave it a character string. Specifying a
>column in your data might tell you something more interesting... e.g.
>str( df2_TZ$DateTimeStamp )
>If that says you have character data then Jim Lemon's suggestion would
>be a good next thing to look at. If it is factor data then you should
>use the as.character function on the data column and then follow Jim's
>suggestion. If it is numeric then you probably need to convert it using
>an appropriate origin (e.g. as described at [1] or [2]).
>I have had best luck setting the default timezone string when
>converting to POSIXt types... e.g.
># specify timezone assumed by input data
>Sys.setenv( TZ="GMT" )
>testdtm <- as.POSIXct( "1/1/2016 00:00", format = "%m/%d/%Y %H:%M" )
># inspect the result
>str( testdtm )
># view data from a different timezone
>Sys.setenv( TZ="Etc/GMT+8" )
># no change to the underlying data, but it prints out differently now
>because the tz attribute is "" which implies using the default TZ
>[1] http://blog.mollietaylor.com/2013/08/date-formats-in-r.html
>[2] https://www.r-project.org/doc/Rnews/Rnews_2004-1.pdf
>Sent from my phone. Please excuse my brevity.
>On February 19, 2016 7:48:31 AM PST, D Wolf <doug45290 at yahoo.com>
>Hello Jeff,
>I ran str() on the vector and it returned character.>
>str("DF_exp.xlsx") chr "DF_exp.xlsx"
>This is my first thread on this forum, and I'm not sure how to reply to
>the thread instead of just sending the reply to your email account; I
>don't see a 'reply' link in the thread.I've read this page and I don't
>think it advises on how to reply in the thread: R: Posting Guide: How
>to ask good questions that prompt useful answers
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>Thank You,Doug Wolfinger
>On Friday, February 19, 2016 12:51 AM, Jeff Newmiller
><jdnewmil at dcn.davis.ca.us> wrote:
>You are being rather scattershot in your explanation, so I suspect you
>are not being systematic in your troubleshooting. Use the str function
>to examine the data column after you pull it in from excel. It may be
>numeric, factor, or character, and the approach depends on which that
>function returns. 
>Sent from my phone. Please excuse my brevity.
>On February 18, 2016 1:12:40 PM PST, D Wolf via R-help
><r-help at r-project.org> wrote:
>Hello,I am trying to read a data frame column named DateTimeStamp. The
>time is in GMT in this format: 1/4/2013 23:30
>df2_TZ = read.xlsx2("DF_exp.xlsx", sheetName = "Sheet1")
>It's good to that line. But these three lines, which makes the
>dataframe, converts the column's values to NA:df2_TZ$DateTimeStamp =
>as.POSIXct(df2_TZ$DateTimeStamp, format="%m/%d/%Y %H:%M:%S", tz="GMT")
>and... df2_TZ$DateTimeStamp =
>as.POSIXct(as.character(df2_TZ$DateTimeStamp), format = "%m/%d/%Y
>and...df2_TZ$DateTimeStamp = as.Date(df2_TZ$DateTimeStamp, format =
>"%m/%d/%Y %H:%M:%S")
>This line returns and error...df2_TZ$DateTimeStamp =
>as.POSIXct(as.Date(df2_TZ$DateTimeStamp), format = "%m/%d/%Y %H:%M:%S")
>"Error in charToDate(x) :  
>character string is not in a standard unambiguous format"
>Additionally, I need to convert from GMT to North American time zones,
>and I think the advice on this page would
>be good for
>that: http://blog.revolutionanalytics.com/2009/06/converting-time-zones.html
>My ultimate goal is to write an R program that finds data in another
>variable in df2_TZ that corresponds to a date and time that match up
>with the date and time in another data frame. For now, any help reading
>the column would be much appreciated.
>Thank You,Doug
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