How do I convert date to number of days in R

Question

How do I convert date to number of days, starting from the first day of the year.

How do I convert the following to the expected result below?

   Dat         


        

Answer 1

Assuming that you wish to count January 1 of the year as 0 we get:

DF <- data.frame(Date = c("02/01/2000", "20/02/2000", "12/12/2000", "13/01/2001"))
DF$Date <- as.Date(DF$Date, "%d/%m/%Y")

Diff <- function(x, start) as.numeric(x - as.Date(cut(start, "year")))
transform(DF, NumDays = Diff(Date, Date), TotalDays = Diff(Date, Date[1]))

which gives;

        Date NumDays TotalDays
1 2000-01-02       1         1
2 2000-02-20      50        50
3 2000-12-12     346       346
4 2001-01-13      12       378

If you want to count January 1st as 1 then add 1 to the expression in Diff.

UPDATE: Correction.

UPDATE: Added DFdefinition to make it self contained.

UPDATE: We add a run using data in comment below.

> DF <- data.frame(Date = as.Date(c("1980-01-03", "1980-01-04", "1980-01-05", 
+ "1980-01-07", "1980-01-10", "1980-01-16")))
> 
> Diff <- function(x, start) as.numeric(x - as.Date(cut(start, "year")))
> transform(DF, NumDays = Diff(Date, Date), TotalDays = Diff(Date, Date[1]))
        Date NumDays TotalDays
1 1980-01-03       2         2
2 1980-01-04       3         3
3 1980-01-05       4         4
4 1980-01-07       6         6
5 1980-01-10       9         9
6 1980-01-16      15        15