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

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