[R] as.Date() clarification

[Achim Zeileis]

On Thu, 3 Jul 2008, Ron Michael wrote:

> When I use the code as.Date(0) the I get : "1970-01-01". My question is 
> why it is like that? help file on as.Date seems to give no light on 
> that. Can anyone clarify me?

In R, "Date" objects are stored internally as the number of days since 
1970-01-01 (typically the epoch on Unix systems):

## today
dd <- as.Date("2008-07-03")
dd

## number of days since 1970-01-01
dn <- as.numeric(dd)
dn

## In base R, you can reverse this if you specify the origin used
as.Date(dn, origin = "1970-01-01")

## If you load package "zoo", this is made the default
library("zoo")
as.Date(dn)

The idea of the zoo implementation is that you can switch back and forth 
between the "Date" and numeric representation. The implementation in base 
R, on the other hand, wants to be agnostic to the origin used and hence 
sets no default for that argument.

hth,
Z


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