Extract Date in R

Question

I struggle mightily with dates in R and could do this pretty easily in SPSS, but I would love to stay within R for my project.

I have a date column in my data fram

Answer 1

> d = as.Date("2003-10-09", format="%Y-%m-%d")
> format(d, "%m-%d")
[1] "10-09"

Answer 2

Is this what you are looking for?

library(ggplot2)
## make up data for two seasons a and b
a = as.Date("2010/10/1")
b = as.Date("2011/10/1")
a.date <- seq(a, by='1 week', length=28)
b.date <- seq(b, by='1 week', length=28)

## make up some score data  
a.score <- abs(trunc(rnorm(28, mean = 10, sd = 5)))
b.score <- abs(trunc(rnorm(28, mean = 10, sd = 5)))

## create a data frame   
df <- data.frame(a.date, b.date, a.score, b.score)
df

## Since I am using ggplot I better create a "long formated" data frame
df.molt <- melt(df, measure.vars = c("a.score", "b.score"))
levels(df.molt$variable) <- c("First season", "Second season")
df.molt

Then, I am using ggplot2 for plotting the data:

## plot it
ggplot(aes(y = value, x = a.date), data = df.molt) + geom_point() +   
geom_line() + facet_wrap(~variable, ncol = 1) + 
scale_x_date("Date", format = "%m-%d")

If you want to modify the x-axis (e.g., display format), then you'll probably be interested in scale_date.

Answer 3

You have to remember Date is a numeric format, representing the number of days passed since the "origin" of the internal date counting :

> str(Date)
Class 'Date'  num [1:10] 14245 14360 14475 14590 14705 ...

This is the same as in EXCEL, if you want a reference. Hence the solution with format as perfectly valid.

Now if you want to set the first date of a year as October 1st, you can construct some year index like this :

redefine.year <- function(x,start="10-1"){
  year <- as.numeric(strftime(x,"%Y"))
  yearstart <- as.Date(paste(year,start,sep="-"))

  year + (x >= yearstart) - min(year) + 1
}

Testing code :

Start <- as.Date("2009-1-1")    
Stop <- as.Date("2011-11-1")
Date <- seq(Start,Stop,length.out=10)

data.frame( Date=as.character(Date),
            year=redefine.year(Date))

gives

         Date year
1  2009-01-01    1
2  2009-04-25    1
3  2009-08-18    1
4  2009-12-11    2
5  2010-04-05    2
6  2010-07-29    2
7  2010-11-21    3
8  2011-03-16    3
9  2011-07-09    3
10 2011-11-01    4