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 frame and want to remove the year completely in order to leave the month and day. Here is a peak at my original data.

> head(ds$date)
[1] "2003-10-09" "2003-10-11" "2003-10-13" "2003-10-15" "2003-10-18" "2003-10-20"
> class((ds$date))
[1] "Date"

I "want" it to be.

> head(ds$date)
[1] "10-09" "10-11" "10-13" "10-15" "10-18" "10-20"
> class((ds$date))
[1] "Date"

If possible, I would love to set the first date to be October 1st instead of January 1st.

Any help you can provide will be greatly appreciated.

EDIT: I felt like I should add some context. I want to plot an NHL player's performance over the course of a season which starts in October and ends in April. To add to this, I would like to facet the plots by each season which is a separate column in my data frame. Because I want to compare cumulative performance over the course of the season, I believe that I need to remove the year portion, but maybe I don't; as I indicated, I struggle with dates in R. What I am looking to accomplish is a plot that compares cumulative performance over relative dates by season and have the x-axis start in October and end in April.

Solution 1

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.

Solution 2

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

Solution 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

Comments

[Btibert3]

thanks, but that stores it as a character. Can I put that result back into date form?

[Hong Ooi]

@Btibert3: You should distinguish between what the date variable contains, and how it's displayed. If you want to hide the year for display purposes, the code above does the job, but the date itself stays the same.

[Jay]

Great answer. There are many flexible date formatting options in R for extraction and display of the date compoents in various ways. See the help for format.

[Btibert3]

Almost! The only thing that I need is that the left side of the graph should start with October because that is when the season starts. Said differently, the axis should range from October to April when read left to right. That is the part that I am getting hung up on. thx for your help!