[R] 'as.Date' conversion of classes POSIX*t (problem/feature)?

From: Angel Spassov <anspassov_at_googlemail.com>
Date: Fri, 04 Jul 2008 00:48:12 +0200


Hi,

I'm working with objects of classes "Date","POSIXlt" and "POSIXct" and still having some Date/Time-related concepts unclear. In the documentation of "as.Date" one can find:

"The 'as.Date' methods accept ... '"POSIXlt"' and '"POSIXct"'. (The last are converted to days by ignoring the time after midnight in the representation of the time in UTC.)"

As far as why "POSIXct" ignores the time after midnight is still open question for me and some basic hints about why this happens would be greatly appreciated? Having mentioned this one can avoid an eventual confusion when interpreting the following:

 x1 = as.POSIXlt("2008-01-02 00:58:00")

x2 = as.POSIXct("2008-01-02 00:58:00")

as.Date(x1); as.Date(x2)

[1] "2008-01-02"

[1] "2008-01-01"

 which happened to me at the beginning. Aside from this, my real problem was with the (very) unexpected behavior of the 'as.data.frame'-function which seems to coerce the class '"POSIXlt"' to class '"POSIXct"' and consequently making my life considerably harder through:

as.Date(as.data.frame(x1)[1,1])

[1] "2008-01-01"

where

as.Date(x1)

[1] "2008-01-02"

Can anyone offer me a strong evidence that exactly this behaviour of R gives me advantage over the "naturally" expected conversion to "2008-01-02"? Thanks in advance.

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