strptime, as.POSIXct and as.Date return unexpected 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 0.9

Answer 1

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"

Answer 2

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