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"

Oviposit

Could this be related to the time zone? What if you supply a specific time zone when calling as.POSIXct(d, tz = "...")? – Banky

@beginneR I wanted to know the reason why that happens, I already have a workaround. – Oviposit

Maybe because of this (from ?as.POSIXct): "Dates without times are treated as being at midnight UTC." – Banky

What is a fromat? :) – Auden

Answer 1

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"

Auden

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

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") – Oviposit

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