[R] Date read correctly from CSV, then reformatted incorrectly by R

[Eric Berger]

Hi Philip,
This is a recurring question and there are many ways to do this.
My preference is to use the lubridate package.

library(lubridate)
a <- "15/01/2010"
b <- dmy(a)
b
# "2010-01-15"
class(b)
# [1] "Date"

HTH,
Eric


On Sat, Nov 20, 2021 at 7:09 PM Philip Monk <prmonk using gmail.com> wrote:

> Hello,
>
> Simple but infuriating problem.
>
> Reading in CSV of data using :
>
> ```
> # CSV file has column headers with date of scene capture in format
> dd/mm/yyyy
> # check.names = FALSE averts R incorrectly processing dates due to '/'
> data <- read.csv("C:/R_data/Bungala (b2000) julian.csv", check.names =
> FALSE)
>
> # Converts data table from wide (many columns) to long (many rows) and
> creates the new object 'data_long'
> # Column 1 is the 'Buffer' number (100-2000), Columns 2-25 contain monthly
> data covering 2 years (the header row being the date, and rows 2-21 being a
> value for each buffer).
> # Column headers for columns 2:25 are mutated into a column called 'Date',
> values for each buffer and each date into the column 'LST'
> data_long <- data %>% pivot_longer(cols = 2:25, names_to = "Date",
> values_to = "LST")
>
> # Instructs R to treat the 'Date' column data as a date
> data_long$Date <- as.Date(data_long$Date)
> ```
>
> Using str(data), I can see that R has correctly read the dates in the
> format %d/%m/%y (e.g. 15/12/2015) though has the data type as chr.
>
> Once changing the type to 'Date', however, the date is reconfigured.  For
> instance, 15/01/2010 (15 January 2010), becomes 0015-01-20.
>
> I've tried ```data_long$Date <- as.Date(data_long$Date, format =
> "%d/%m.%y")```, and also ```tryformat c("%d/%m%y")```, but either the error
> persists or I get ```NA```.
>
> How do I make R change Date from 'chr' to 'date' without it going wrong?
>
> Suggestions/hints/solutions would be most welcome.  :)
>
> Thanks for your time,
>
> Philip
>
> Part-time PhD Student (Environmental Science)
> Lancaster University, UK.
>
> ~~~~~
>
> I asked a question a few weeks ago and put together the answer I needed
> from the responses but didn't know how to say thanks on this list.  So,
> thanks Andrew Simmons, Bert Gunter, Jeff Newmiller and Daniel Nordlund!
>
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>
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