Please consider the following
$ R --vanilla
> as.Date("01 Jan 2000")
Error in charToDate(x) :
character string is not in a standard unambiguous format
R version 2.15.2 (2012-10-26)
Platform: x86_64-w64-mingw32/x64 (64-bit)
 LC_COLLATE=English_United Kingdom.1252
 LC_CTYPE=English_United Kingdom.1252
 LC_MONETARY=English_United Kingdom.1252
 LC_TIME=English_United Kingdom.1252
attached base packages:
 stats graphics grDevices utils datasets methods base
This is documented behavior. From
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
?strptime. 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