strptime、as.POSIXct 和 as.Date 返回意外的 NA

Question

When I try to parse a timestamp in the following format: "Thu Nov 8 15:41:45 2012", only NA is returned.

I am using Mac OS X, R 2.15.2 and Rstudio 0.97.237. The language of my OS is Dutch: I presume this has something to do with it.

When I try strptime , NA is returned:

var <- "Thu Nov 8 15:41:45 2012"
strptime(var, "%a %b %d %H:%M:%S %Y")
# [1] NA

Neither does as.POSIXct work:

as.POSIXct(var, "%a %b %d %H:%M:%S %Y")
# [1] NA

I also tried as.Date on the string above but without %H:%M:%S components:

as.Date("Thu Nov 8 2012", "%a %b %d %Y")
# [1] NA

Any ideas what I could be doing wrong?

Answer 1

I think it is exactly as you guessed, strptime fails to parse your date-time string because of your locales. Your string contains both abbreviated weekday ( %a ) and abbreviated month name ( %b ). These time specifications are described in ?strptime :

Details

%a : Abbreviated weekday name in the current locale on this platform

%b : Abbreviated month name in the current locale on this platform .

"Note that abbreviated names are platform-specific (although the standards specify that in the C locale they must be the first three letters of the capitalized English name:"

"Knowing what the abbreviations are is essential if you wish to use %a , %b or %h as part of an input format: see the examples for how to check."

See also

[...] locales to query or set a locale.

The issue of locales is relevant also for as.POSIXct , as.POSIXlt and as.Date .

From ?as.POSIXct :

Details

If format is specified, remember that some of the format specifications are locale-specific, and you may need to set the LC_TIME category appropriately via Sys.setlocale . This most often affects the use of %b , %B (month names) and %p (AM/PM).

From ?as.Date :

Details

Locale-specific conversions to and from character strings are used where appropriate and available. This affects the names of the days and months.

---

Thus, if weekdays and month names in the string differ from those in the current locale, strptime , as.POSIXct and as.Date fail to parse the string correctly and NA is returned.

However, you may solve this issue by changing the locales :

# First save your current locale
loc <- Sys.getlocale("LC_TIME")

# Set correct locale for the strings to be parsed
# (in this particular case: English)
# so that weekdays (e.g "Thu") and abbreviated month (e.g "Nov") are recognized
Sys.setlocale("LC_TIME", "en_GB.UTF-8")
# or
Sys.setlocale("LC_TIME", "C") 

#Then proceed as you intended
x <- "Thu Nov 8 15:41:45 2012" 
strptime(x, "%a %b %d %H:%M:%S %Y")
# [1] "2012-11-08 15:41:45"

# Then set back to your old locale
Sys.setlocale("LC_TIME", loc) 

With my personal locale I can reproduce your error:

Sys.setlocale("LC_TIME", loc)
# [1] "fr_FR.UTF-8"

strptime(var,"%a %b %d %H:%M:%S %Y")
# [1] NA

Answer 2

Was just messing around with same problem, and found this solution to be much cleaner because there is no need to change any of system settings manually, because there is a wrapper function doing this job in the lubridate package, and all you have to do is set the argument locale :

date <- c("23. juni 2014", "1. november 2014", "8. marts 2014", "16. juni 2014", "12. december 2014", "13. august 2014")
df$date <- dmy(df$Date, locale = "Danish")
[1] "2014-06-23" "2014-11-01" "2014-03-08" "2014-06-16" "2014-12-12" "2014-08-13"