R for Journalists > Wrangling data > Dealing with dates

Dealing with dates

Dates come in as characters, most of the time.

You’ll need to convert them into a date variable

We’ll be using the lubridate package.

Here’s an example of a character variable that might be in a data frame.

some_date <- "12-31-1999"

Convert that date into a date variable with the function mdy()

# If you don't have lubridate installed yet uncomment the line below and run it
#install.packages("lubridate")

# NOTE: IF YOU GET AN ERROR ABOUTZ NOT HAVING A PACKAGE CALLED stringi
# UNCOMMENT AND RUN THE LINES BELOW IF YOU HAVE A WINDOWS MACHINE

#install.packages("glue", type="win.binary")
#install.packages("stringi", type="win.binary")
#install.packages("stringr", type="win.binary")
#install.packages("lubridate", type="win.binary")

# UNCOMMENT AND RUN THE LINES BELOW IF YOU HAVE A MAC MACHINE

#install.packages("glue", type="mac.binary")
#install.packages("stringi", type="mac.binary")
#install.packages("stringr", type="mac.binary")
#install.packages("lubridate", type="mac.binary")

library(lubridate)

mdy(some_date)

## [1] "1999-12-31"

The mdy() function is very versatile. It stand for month-date-year.

And it’ll be able to parse any version of that (with slashes or commas, or dashes) as long as that’s the order of the information.

Check it out:

data <- data.frame(First=c("Charlie", "Lucy", "Peppermint"),
                   Last=c("Brown", "van Pelt", "Patty"),
                   birthday=c("10-31-06", "2/4/2007", "June 1, 2005"))

data$DOB <- mdy(data$birthday)

data

##        First     Last     birthday        DOB
## 1    Charlie    Brown     10-31-06 2006-10-31
## 2       Lucy van Pelt     2/4/2007 2007-02-04
## 3 Peppermint    Patty June 1, 2005 2005-06-01

Reading dates

Order of elements in date-time	Parse function
year, month, day	`ymd()`
year, day, month	`ydm()`
month, day, year	`mdy()`
day, month, year	`dmy()`
hour, minute	`hm()`
hour, minute, second	`hms()`
year, month, day, hour, minute, second	`ymd_hms()`

Accessing date parts

Date component	Function
Year	`year()`
Month	`month()`
Week	`week()`
Day of year	`yday()`
Day of month	`mday()`
Day of week	`wday()`
Hour	`hour()`
Minute	`minute()`
Second	`ymd_hms()`
Time zone	`ymd_hms()`

Now that we have the date in the right format, we can extract data from it with the functions above.

data$year <- year(data$DOB)
data$month <- month(data$DOB, label=TRUE)
data$day <- day(data$DOB)
data$weekday <- wday(data$DOB, label=TRUE, abbr=FALSE)

data

##        First     Last     birthday        DOB year month day   weekday
## 1    Charlie    Brown     10-31-06 2006-10-31 2006   Oct  31   Tuesday
## 2       Lucy van Pelt     2/4/2007 2007-02-04 2007   Feb   4    Sunday
## 3 Peppermint    Patty June 1, 2005 2005-06-01 2005   Jun   1 Wednesday

Date arithmetic

The function difftime() extracts the number of days between two dates that are passed to it

# We're going to use the now() function which brings in the date for today

today <- now()
data$age <- difftime(today, data$DOB)

data

##        First     Last     birthday        DOB year month day   weekday
## 1    Charlie    Brown     10-31-06 2006-10-31 2006   Oct  31   Tuesday
## 2       Lucy van Pelt     2/4/2007 2007-02-04 2007   Feb   4    Sunday
## 3 Peppermint    Patty June 1, 2005 2005-06-01 2005   Jun   1 Wednesday
##             age
## 1 4449.774 days
## 2 4353.774 days
## 3 4966.774 days

And how does that translate into years?

With some math. We’ll have to turn the column into a number, first.

data$age_years <- as.numeric(data$age) / 365.25 #.25 because of leap years

data

##        First     Last     birthday        DOB year month day   weekday
## 1    Charlie    Brown     10-31-06 2006-10-31 2006   Oct  31   Tuesday
## 2       Lucy van Pelt     2/4/2007 2007-02-04 2007   Feb   4    Sunday
## 3 Peppermint    Patty June 1, 2005 2005-06-01 2005   Jun   1 Wednesday
##             age age_years
## 1 4449.774 days  12.18282
## 2 4353.774 days  11.91998
## 3 4966.774 days  13.59829

That’s a pretty good start for now. To see more functions and examples, check out the vignette for lubridate.

Your turn

Challenge yourself with these exercises so you’ll retain the knowledge of this section.

Instructions on how to run the exercise app are on the intro page to this section.