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"

Could this be related to the time zone? What if you supply a specific time zone when calling `as.POSIXct(d, tz = "...")`? — talat, Oct 09 '14 at 13:35
@beginneR I wanted to know the reason why that happens, I already have a workaround. — Carlos Cinelli, Oct 09 '14 at 13:38
Maybe because of this (from `?as.POSIXct`): "Dates without times are treated as being at midnight UTC." — talat, Oct 09 '14 at 13:45

score 24 · Accepted Answer · answered Oct 09 '14 at 14:02

24

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"

answered Oct 09 '14 at 14:02

Joshua Ulrich

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...or convert the Date to `character` first: `as.POSIXct(format(d))` – GSee Oct 09 '14 at 14:03
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")` – Carlos Cinelli Oct 09 '14 at 17:16
1

@CarlosCinelli: I'm not sure that solves it, because that is midnight UTC, not midnight local time. – Joshua Ulrich Oct 09 '14 at 17:19

Convert Date to POSIXct

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