R: Why is class Date lost upon subsetting - r

Question

Here is an easy example. I have a a data frame with three dates in it:
Data <- as.data.frame(as.Date(c('1970/01/01', '1970/01/02', '1970/01/03')))
names(Data) <- "date"
Now I add a column consisting of the same entries:
for(i in 1:3){
Data[i, "date2"] <- Data[i, "date"]
}
Output looks like this:
date date2
1 1970-01-01 0
2 1970-01-02 1
3 1970-01-03 2
For unknown reasons the class of column date2 is numeric instead of date which was the class of date. Curiously, if you tell R explicitly to use the Date format:
for(i in 1:3){
Data[i, "date3"] <- as.Date(Data[i, "date"])
}
it doesn't make any difference.
date date2 date3
1 1970-01-01 0 0
2 1970-01-02 1 1
3 1970-01-03 2 2
The problem seems to be in the use of subsetting [], in more interesting examples where you have two columns of dates and want to create a third one that picks a date from one of the two other columns depending on some factor the same happens.
Of course we can fix everything in retrospect by doing something like:
Data$date4 <- as.Date(Data$date2, origin = "1970-01-01")
but I'm still wondering: why? Why is this happening? Why can't my dates just stay dates when being transferred to another column??

Answer 1

This is not a final solution, but I think that can help to understand.
Here your data :
Data <- data.frame(date =
as.Date(c('2000/01/01', '2012/01/02', '2013/01/03')))
Take this 2 vectors , one typed by default as numeric and the second as Date.
vv <- vector("numeric",3)
vv.Date <- vector("numeric",3)
class(vv.Date) <- 'Date'
vv
[1] 0 0 0
> vv.Date
[1] "1970-01-01" "1970-01-01" "1970-01-01" ## type dates is initialized by the origin 01-01-1970
Now if I try to assign the first element of each vector as you do in the first step of your loop:
vv[1] <- Data$date[1]
vv.Date[1] <- Data$date[1]
vv
[1] 10957 0 0
> vv.Date
[1] "2000-01-01" "1970-01-01" "1970-01-01"
As you see the typed vector is well created. What happen, when you assign a vector by a scalar value , R try internally to convert it to the type of the vector. To return to your example, When you do this :
You a creating a numeric vector (vv), and you try to assign dates to it:
for(i in 1:3){
Data[i, "date3"] <- as.Date(Data[i, "date"])
}
If you type your date3 , for example:
Data$date3 <- vv.Date
then you try again
for(i in 1:3){
Data[i, "date3"] <- as.Date(Data[i, "date"])
}
You will get a good result:
date date3
1 2000-01-01 2000-01-01
2 2012-01-02 2012-01-02
3 2013-01-03 2013-01-03