Changing date format in R

date, format, r, r-faq, strptime

Solution

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.

Problem

I have some very simple data in R that needs to have its date format changed: ``` date midpoint 1 31/08/2011 0.8378 2 31/07/2011 0.8457 3 30/06/2011 0.8147 4 31/05/2011 0.7970 5 30/04/2011 0.7877 6 31/03/2011 0.7411 7 28/02/2011 0.7624 8 31/01/2011 0.7665 9 31/12/2010 0.7500 10 30/11/2010 0.7734 11 31/10/2010 0.7511 12 30/09/2010 0.7263 13 31/08/2010 0.7158 14 31/07/2010 0.7110 15 30/06/2010 0.6921 16 31/05/2010 0.7005 17 30/04/2010 0.7113 18 31/03/2010 0.7027 19 28/02/2010 0.6973 20 31/01/2010 0.7260 21 31/12/2009 0.7154 22 30/11/2009 0.7287 23 31/10/2009 0.7375 ``` Rather than `%d/%m/%Y`, I would like it in the standard R format of `%Y-%m-%d` How can I make this change? I have tried: ``` nzd$date <- format(as.Date(nzd$date), "%Y/%m/%d") ``` But that just cut off the year and added zeros to the day: ``` [1] "0031/08/20" "0031/07/20" "0030/06/20" "0031/05/20" "0030/04/20" [6] "0031/03/20" "0028/02/20" "0031/01/20" "0031/12/20" "0030/11/20" [11] "0031/10/20" "0030/09/20" "0031/08/20" "0031/07/20" "0030/06/20" [16] "0031/05/20" "0030/04/20" "0031/03/20" "0028/02/20" "0031/01/20" [21] "0031/12/20" "0030/11/20" "0031/10/20" "0030/09/20" "0031/08/20" [26] "0031/07/20" "0030/06/20" "0031/05/20" "0030/04/20" "0031/03/20" [31] "0028/02/20" "0031/01/20" "0031/12/20" "0030/11/20" "0031/10/20" [36] "0030/09/20" "0031/08/20" "0031/07/20" "0030/06/20" "0031/05/20" ``` Thanks!

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