drRussClay drRussClay - 2 months ago 9
R Question

In R, as.Date is being returned as type 'double'

I'm having some trouble working with the as.Date function in R. I have a vector of dates that I'm reading in from a .csv file that are coming in as a factor of integers or as character (depending on how I read in the file, but this doesn't seem to have anything to do with the issue), formatted as %m/%d/%Y.

I'm going through the file row by row, pulling out the date field and trying to convert it for use elsewhere using the following code:

tmpDtm<-as.Date(as.character(tempDF$myDate), "%m/%d/%Y")

This seems to give me what I want, for example, if I do this to a starting value of 12/30/2014, I get the value "2014-12-30" returned. However, if I examine this value using typeof(), R tells me that it its data type is 'double'. Additionally, if I try to bind this to other values and store it in a data frame using c() or cbind(), in the data frame, it winds up being stored as 16434, which looks to me like some sort of different internal storage value of a date. I'm pretty sure that's what it is too because if I try to convert that value again using as.Date(), it throws an error asking for an origin.

So, two questions: Is this as expected? If so, is there a more appropriate way to convert a date so that I actually end up with a date-typed object?

Thank you

Answer

Dates are internally represented as double, as you can see in the following example:

> typeof(as.Date("09/12/16", "%m/%d/%y"))
[1] "double"

it is still marked a class Date, as in

> class(as.Date("09/12/16", "%m/%d/%y"))
[1] "Date"

and because it is a double, you can do computations with it. But because it is of class Date, these computations lead to Dates:

> as.Date("09/12/16", "%m/%d/%y") + 1
[1] "2016-09-13"
> as.Date("09/12/16", "%m/%d/%y") + 31
[1] "2016-10-13"

EDIT I have asked for c() and cbind(), because they can be assciated with some strange behaviour. See the following example, where switching the order within c changes not the type but the class of the result:

> c(as.Date("09/12/16", "%m/%d/%y"), 1)
[1] "2016-09-12" "1970-01-02"
> c(1, as.Date("09/12/16", "%m/%d/%y"))
[1]     1 17056

> class(c(as.Date("09/12/16", "%m/%d/%y"), 1))
[1] "Date"
> class(c(1, as.Date("09/12/16", "%m/%d/%y")))
[1] "numeric"
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