R returns NA when I try to coerce a string to date using as.Date

Question

I am trying to convert "March 15, 2017" to date. as.Date("March 15, 2017", "%B %d, %Y") and it returned NA I feel that the syntax fits well, what is the problem?

Answer 1

You are close, but have been bitten by your locale. If you look at the documentation for strptime, you will notice that

%B   Full month name in the current locale. (Also matches abbreviated name on input.)

This is also the case for my system since Slovenian doesn't have English month names:

> as.Date("March 15, 2017", "%B %d, %Y")
[1] NA

> Sys.getlocale()
[1] "LC_COLLATE=Slovenian_Slovenia.1250;LC_CTYPE=Slovenian_Slovenia.1250;LC_MONETARY=Slovenian_Slovenia.1250;LC_NUMERIC=C;LC_TIME=Slovenian_Slovenia.1250"

What you can do is change locale, perhaps only for the duration of the conversion.

> Sys.setlocale(locale = "English")
[1] "LC_COLLATE=English_United States.1252;LC_CTYPE=English_United States.1252;LC_MONETARY=English_United States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252"
> as.Date("March 15, 2017", "%B %d, %Y")
[1] "2017-03-15"

And then back to normal

> Sys.setlocale(locale = "Slovenian")
[1] "LC_COLLATE=Slovenian_Slovenia.1250;LC_CTYPE=Slovenian_Slovenia.1250;LC_MONETARY=Slovenian_Slovenia.1250;LC_NUMERIC=C;LC_TIME=Slovenian_Slovenia.1250"
> as.Date("March 15, 2017", "%B %d, %Y")
[1] NA

But if I use a Slovenian name for March:

> as.Date("Marec 15, 2017", "%B %d, %Y")
[1] "2017-03-15"

Locale name will depend on your operating system, see ?Sys.setlocale for more info.