Problem with changing format of date and time in R

Question

I am having a problem with date & time format in R.

My R code is having the format: "%Y%m%d %H%M%OS". For example: "20170929 20:59:56.149"

Time in my case is character variable.

I am trying to write a R code where type of format will be converted into the format "%Y-%m-%d %H:%M:%OS"

(data <- strptime(data$time, 
                    format = "%Y-%m-%d  %H:%M:%OS", 
                    tz = "GMT")) 

or for example

data$time <- as.POSIXct(data$time, format = "%Y-%m-%d  %H:%M:%OS")

But, I am getting NA. What could this be caused by? Do you have any idea how to fix it? Thank you in advance for your help!

Answer 1

From the manual help(strftime): If the specified time is invalid (for example ‘"2010-02-30 08:00"’) all the components of the result are ‘NA’.

Furthermore, if you want to manipulate your output format of a date you need the date stored as a Date-class (illustration of @izyda's comments).

First, reformat your date to make it easier to manipulate:

data <- "20170929 20:59:56.149"
dat_new <- paste( paste( substr(data, 1, 4), 
   substr(data, 5, 6), substr(data, 7, 8), sep="-" ), 
   substr(data, 10,nchar(data)) )
dat_new
[1] "2017-09-29 20:59:56.149"

Then, change the class to Date:

dat_cor <- as.POSIXct( dat_new, tz="GMT" )
dat_cor
[1] "2017-09-29 20:59:56 GMT"
class(dat_new)
[1] "character"
class(dat_cor)
[1] "POSIXct" "POSIXt"

Finally, choose your output format:

strftime( dat_cor, format="%m/%d/%Y %H:%M:%OS3", tz="GMT" )
[1] "09/29/2017 20:59:56.148"
# or
strftime( dat_cor, format="%Y-%d-%m %H:%M:%OS3", tz="GMT" )
[1] "2017-29-09 20:59:56.148"

Answer 2

I think @izyda has already cleared up the confusion. The format that you include in strptime is the format of the data that you have, so in this case you use -

x <- "20170929 20:59:56.149"
strptime(x, format = "%Y%m%d  %H:%M:%OS", tz = "GMT")
#[1] "2017-09-29 20:59:56 GMT"

The same can be applied to as.POSIXct.

Alternatively, if you are confused by different formats or tired of referring to ?strptime you can use lubridate package. In this case, function ymd_hms would help.

lubridate::ymd_hms(x)
#[1] "2017-09-29 20:59:56 UTC"