What are the "standard unambiguous date" formats for string-to-date conversion in R?

Please consider the following

$ R --vanilla

> as.Date("01 Jan 2000")
Error in charToDate(x) :
    character string is not in a standard unambiguous format

But that date clearly is in a standard unambiguous format. Why the error message?

Worse, an ambiguous date is apparently accepted without warning or error and then read incorrectly!

> as.Date("01/01/2000")
[1] "0001-01-20"

I've searched and found 28 other questions in the [R] tag containing this error message. All with solutions and workarounds involving specifying the format, iiuc. This question is different in that I'm asking where are the standard unambiguous formats defined anyway, and can they be changed? Does everyone get these messages or is it just me? Perhaps it is locale related?

In other words, is there a better solution than needing to specify the format?

29 questions containing "[R] standard unambiguous format"

> sessionInfo()
R version 2.15.2 (2012-10-26)
Platform: x86_64-w64-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=English_United Kingdom.1252
[2] LC_CTYPE=English_United Kingdom.1252
[3] LC_MONETARY=English_United Kingdom.1252
[4] LC_NUMERIC=C
[5] LC_TIME=English_United Kingdom.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

This is documented behavior. From ?as.Date:

> format: A character string. If not specified, it will try > '"%Y-%m-%d"' then '"%Y/%m/%d"' on the first non-'NA' element, > and give an error if neither works.

as.Date("01 Jan 2000") yields an error because the format isn't one of the two listed above. as.Date("01/01/2000") yields an incorrect answer because the date isn't in one of the two formats listed above.

I take "standard unambiguous" to mean "ISO-8601" (even though as.Date isn't that strict, as "%m/%d/%Y" isn't ISO-8601).

If you receive this error, the solution is to specify the format your date (or datetimes) are in, using the formats described in the Details section in ?strptime.

Make sure that the order of the conversion specification as well as any separators correspond exactly with the format of your input string. Also, be sure to use particular care if your data contain day/month names and/or abbreviations, as the conversion will depend on your locale (see the examples in ?strptime and read ?LC_TIME; see also strptime, as.POSIXct and as.Date return unexpected NA).

> In other words, is there a better solution than needing to specify the format?

Yes, there is now (ie in late 2016), thanks to anytime::anydate from the anytime package.

See the following for some examples from above:

R> anydate(c("01 Jan 2000", "01/01/2000", "2015/10/10"))
[1] "2000-01-01" "2000-01-01" "2015-10-10"
R> 

As you said, these are in fact unambiguous and should just work. And via anydate() they do. Without a format.

As a complement to @JoshuaUlrich answer, here is the definition of function as.Date.character:

as.Date.character
function (x, format = "", ...) 
{
    charToDate <- function(x) {
        xx <- x[1L]
        if (is.na(xx)) {
            j <- 1L
            while (is.na(xx) && (j <- j + 1L) <= length(x)) xx <- x[j]
            if (is.na(xx)) 
                f <- "%Y-%m-%d"
        }
        if (is.na(xx) || !is.na(strptime(xx, f <- "%Y-%m-%d", 
            tz = "GMT")) || !is.na(strptime(xx, f <- "%Y/%m/%d", 
            tz = "GMT"))) 
            return(strptime(x, f))
        stop("character string is not in a standard unambiguous format")
    }
    res <- if (missing(format)) 
        charToDate(x)
    else strptime(x, format, tz = "GMT")
    as.Date(res)
}
<bytecode: 0x265b0ec>
<environment: namespace:base>

So basically if both strptime(x, format="%Y-%m-%d") and strptime(x, format="%Y/%m/%d") throws an NA it is considered ambiguous and if not unambiguous.

Converting the date without specifying the current format can bring this error to you easily. Here is an example:

sdate <- "2015.10.10"

Convert without specifying the Format:

date <- as.Date(sdate4) # ==> This will generate the same error"""Error in charToDate(x): character string is not in a standard unambiguous format""".

Convert with specified Format:

date <- as.Date(sdate4, format = "%Y.%m.%d") # ==> Error Free Date Conversion.

This works perfectly for me, not matter how the date was coded previously.

library(lubridate)
data$created_date1 <- mdy_hm(data$created_at)
data$created_date1 <- as.Date(data$created_date1)

As a complement: This error can be raised as well if an entry you are trying to cast is a string that should have been NA. If you specify the expected format -or use "real" NAs- there are no problems:

Minimum reproducible example with data.table:

library(data.table)
df <- data.table(date_good = c("01-01-2001", "01-01-2001"), date_bad= ("NA", "01-01-2001"))

df[, .(date_good = as.Date(date_good), date_bad = as.Date(date_bad))]
# Error in charToDate(x) : character string is not in a standard unambiguous format

df[, .(date_good = as.Date(date_good), date_bad = as.Date(date_bad, format="%Y-%m-%d"))]
# No errors; you simply get NA.

df2 <- data.table(date_good = c("01-01-2001", "01-01-2001"), date_bad= (NA, "01-01-2001"))
    
df2[, .(date_good = as.Date(date_good), date_bad = as.Date(date_bad))]
# Just NA

If the date is for example: "01 Jan 2000", I recommend using

library(lubridate)
date_corrected<-dmy("01 Jan 2000")
date_corrected
[1] "2000-01-01"
class(date_corrected)
[1] "Date"

lubridate has a function for almost every type of date.