Convert Date to POSIXct

Question

Why does the Date below change to "2014-07-07" when converted to POSIXct?

Sys.setenv(TZ='America/Sao_Paulo')
d <- as.Date("2014-07-08", format="%Y-%m-%d")
d
[1] "2014-07-08"
as.POSIXct(d)
[1] "2014-07-07 21:00:00 BRT"

Solution

Because as.POSIXct.Date doesn't look for a timezone (and won't pass it to .POSIXct if you specify it in ...) and Date objects are "UTC", so your POSIXct is offset from the UTC of the Date object.

It would be better to call as.POSIXct on the character string directly, if you can:

> as.POSIXct("2014-07-08", format="%Y-%m-%d")
[1] "2014-07-08 BRT"

Comments

[GSee]

...or convert the Date to character first: as.POSIXct(format(d))

[Carlos Cinelli]

Thanks, Joshua. That's strange, there is a (...) argument but it is not passed to .POSIXct, and passing a simple timezone paramater would solve this .POSIXct(unclass(d) * 86400, tz="UTC")

[Joshua Ulrich]

@CarlosCinelli: I'm not sure that solves it, because that is midnight UTC, not midnight local time.