calculating number of days between 2 columns of dates in data frame

Question

I have a data frame which has two columns of dates in the format yyyy/mm/dd. I am trying to calculate the number of days between these two dates for each observation within

Answer 1

In Ronald's example, if the date formats are different (as displayed below) then modify the format parameter

survey <- data.frame(date=c("2012-07-26","2012-07-25"),tx_start=c("2012-01-01","2012-01-01"))

survey$date_diff <- as.Date(as.character(survey$date), format="%Y-%m-%d")-
              as.Date(as.character(survey$tx_start), format="%Y-%m-%d")

survey:

   date      tx_start     date_diff
1 2012-07-26     2012-01-01    207 days
2 2012-07-25     2012-01-01    206 days

Answer 2

You need to use the as.Date formats correctly.

Eg.

x = '2012/07/25'
xd = as.Date(x,'%Y/%m/%d')
xd    # Prints "2012-07-25"

R date formats are similary to *nix ones.

Doing a typeof(xd) shows it as a double ie. days since 1970.

Answer 3

Without your seeing your data (you can use the output of dput(head(survey)) to show us) this is a shot in the dark:

survey <- data.frame(date=c("2012/07/26","2012/07/25"),tx_start=c("2012/01/01","2012/01/01"))

survey$date_diff <- as.Date(as.character(survey$date), format="%Y/%m/%d")-
                  as.Date(as.character(survey$tx_start), format="%Y/%m/%d")
survey
       date   tx_start date_diff
1 2012/07/26 2012/01/01  207 days
2 2012/07/25 2012/01/01  206 days

Answer 4

You could find the difference between dates in columns in a data frame by using the function difftime as follows:

df$diff_in_days<- difftime(df$datevar1 ,df$datevar2 , units = c("days"))

Answer 5

Following Ronald Example I would like to add that it should be considered if the origin and end dates must be included or not in the days count between two dates. I faced the same problem and ended up using a third option with apply. It could be memory inefficient but helps to understand the problem:

   survey <- data.frame(date=c("2012/07/26","2012/07/25"),tx_start=c("2012/01/01","2012/01/01"))

survey$diff_1 <- as.numeric(
  as.Date(as.character(survey$date), format="%Y/%m/%d")-
    as.Date(as.character(survey$tx_start), format="%Y/%m/%d")
)

survey$diff_2<- as.numeric(
  difftime(survey$date ,survey$tx_start , units = c("days"))
)

survey$diff_3 <- apply(X = survey[,c("date", "tx_start")],
                       MARGIN = 1,
                       FUN = function(x)
                         length(
                           seq.Date(
                             from = as.Date(x[2]),
                             to = as.Date(x[1]),
                             by = "day")
                           )
                       )

This gives the following date differences:

        date   tx_start diff_1   diff_2 diff_3
1 2012/07/26 2012/01/01    207 206.9583    208
2 2012/07/25 2012/01/01    206 205.9583    207