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!

The `format` parameter tells the function how to parse the date... When you specify "%Y-%m-%d" - the function looks for those dashes ("-") which do not exist in the example string you gave (with "20170929" your format parameter should be "%Y%m%d"). The same goes for your time parsing. — alexizydorczyk, Oct 21 '21 at 22:54
The core misunderstanding you have, I think, is that the functions `as.Date` or `as. POSIXct` do not simply change the format of a date... they actually try to convert a type (in your case, a character/string) into the `date` or `POSIXct` type. The format parameter tells this conversion function how to interpret the string. Once you have your data represented as a `POSIXct` type, you can then use a different function, `format`, to produce a new string with some desired form. — alexizydorczyk, Oct 21 '21 at 22:56

score 1 · Answer 1 · answered Oct 22 '21 at 06:00

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"

Andre Wildberg · Accepted Answer · 2021-10-22T00:34:51.983

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"

Problem with changing format of date and time in R

2 Answers2