as.Date(as.POSIXct()) gives the wrong date?

Question

I\'d been trying to look through a dataframe extracting all rows where the date component of a POSIXct column matched a certain value.I came across the following which is co

Answer 1

Fellow Australian chiming in here (Brisbane location, Win7 Enterprise 64 bit, R3.0.1):

I can replicate your issue:

> dt <- as.POSIXct('2012-08-06 09:35:23')
> dt
[1] "2012-08-06 09:35:23 EST"
> as.Date(dt)
[1] "2012-08-05"

Since as.Date defaults to UTC (GMT) as listed in ?as.Date:

## S3 method for class 'POSIXct'
as.Date(x, tz = "UTC", ...) 

Forcing the POSIXct representation to UTC then works as expected:

> dt <- as.POSIXct('2012-08-06 09:35:23',tz="UTC")
> as.Date(dt)
[1] "2012-08-06"

Alternatively, matching them both to my local tz works fine too:

> dt <- as.POSIXct('2012-08-06 09:35:23',tz="Australia/Brisbane")
> as.Date(dt,tz="Australia/Brisbane")
[1] "2012-08-06"

Edit: Ambiguity with the EST specification seems to be an issue for me:

Default use of as.POSIXct

> dt.def <- as.POSIXct("2012-01-01 22:00:00")
> dt.def
[1] "2012-01-01 22:00:00 EST"
> as.numeric(dt.def)
[1] 1325419200
> 

Ambiguous EST - should be the same as default

> dt.est <- as.POSIXct("2012-01-01 22:00:00",tz="EST")
> dt.est
[1] "2012-01-01 22:00:00 EST"
> as.numeric(dt.est)
[1] 1325473200
> 

Unambiguous Brisbane, Australia timezone

> dt.bris <- as.POSIXct("2012-01-01 22:00:00",tz="Australia/Brisbane")
> dt.bris
[1] "2012-01-01 22:00:00 EST"
> as.numeric(dt.bris )
[1] 1325419200
> 

Differences

> dt.est - dt.def
Time difference of 15 hours
> dt.est - dt.bris
Time difference of 15 hours
> dt.bris - dt.def
Time difference of 0 secs