Changing date format in R

nzd$date <- format(as.Date(nzd$date), "%Y/%m/%d")

In the above piece of code, there are two mistakes. First of all, when you are reading nzd$date inside as.Date you are not mentioning in what format you are feeding it the date. So, it tries it's default set format to read it. If you see the help doc, ?as.Date you will see

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. Otherwise, the processing is via strptime

The second mistake is: even though you would like to read it in %Y-%m-%d format, inside format you wrote "%Y/%m/%d".

Now, the correct way of doing it is:

> nzd <- data.frame(date=c("31/08/2011", "31/07/2011", "30/06/2011"), 
+                                       mid=c(0.8378,0.8457,0.8147))
> nzd
        date    mid
1 31/08/2011 0.8378
2 31/07/2011 0.8457
3 30/06/2011 0.8147
> nzd$date <- format(as.Date(nzd$date, format = "%d/%m/%Y"), "%Y-%m-%d")
> head(nzd)
        date    mid
1 2011-08-31 0.8378
2 2011-07-31 0.8457
3 2011-06-30 0.8147

There are two steps here:

• Parse the data. Your example is not fully reproducible, is the data in a file, or the variable in a text or factor variable? Let us assume the latter, then if you data.frame is called X, you can do

 X$newdate <- strptime(as.character(X$date), "%d/%m/%Y")

Now the newdate column should be of type Date.

• Format the data. That is a matter of calling format() or strftime():

 format(X$newdate, "%Y-%m-%d")

A more complete example:

R> nzd <- data.frame(date=c("31/08/2011", "31/07/2011", "30/06/2011"), 
+                    mid=c(0.8378,0.8457,0.8147))
R> nzd
        date    mid
1 31/08/2011 0.8378
2 31/07/2011 0.8457
3 30/06/2011 0.8147
R> nzd$newdate <- strptime(as.character(nzd$date), "%d/%m/%Y")
R> nzd$txtdate <- format(nzd$newdate, "%Y-%m-%d")
R> nzd
        date    mid    newdate    txtdate
1 31/08/2011 0.8378 2011-08-31 2011-08-31
2 31/07/2011 0.8457 2011-07-31 2011-07-31
3 30/06/2011 0.8147 2011-06-30 2011-06-30
R> 

The difference between columns three and four is the type: newdate is of class Date whereas txtdate is character.