A.Krueger A.Krueger - 3 months ago 10
R Question

Changing date format in R

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!

Answer

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.

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