Direct conversion between Date and POSIXt classes using the
as.* can give probably undesired results unless great care is
taken with supplying appropriate time zones where needed.
It is generally easier to convert dates between Date and
POSIXt using character formatted dates as an intermediate
representation.
Note that what is described as "not sensible" behavior here is NOT
INCONSISTENT with the documentation for as.POSIXct and
as.POSIXlt.
> # Behavior depends on the timezone of the system
> Sys.setenv(TZ='EST')
> Sys.timezone()
[1] "EST"
>
> # Get some POSIXct times that span 9 hours over an evening and
> # the following morning.
> x <- as.POSIXct('2011-12-10 16:55:26 EST', tz='EST')+(0:9)*3600
>
> # The first 8 date/times are in the evening of Dec 10, the last 2 in the
> # morning of Dec 11 (in EST).
>
> as.character(x)
[1] "2011-12-10 16:55:26" "2011-12-10 17:55:26" "2011-12-10 18:55:26"
[4] "2011-12-10 19:55:26" "2011-12-10 20:55:26" "2011-12-10 21:55:26"
[7] "2011-12-10 22:55:26" "2011-12-10 23:55:26" "2011-12-11 00:55:26"
[10] "2011-12-11 01:55:26"
>
> # Not sensible direction conversion POSIXct->Date.
> # Times after 7pm in the POSIXct object turn up as the next day in
> # the Date Object. (They are interpreted as a time-of-day in GMT
> # and 7pm EST is 12 midnight GMT.)
> as.Date(x)
[1] "2011-12-10" "2011-12-10" "2011-12-10" "2011-12-11" "2011-12-11"
[6] "2011-12-11" "2011-12-11" "2011-12-11" "2011-12-11" "2011-12-11"
>
> # Another way of looking at the as.Date(POSIXct)
>
> data.frame(as.character(x), as.Date(as.character(x)), as.Date(x), check.names=F)
as.character(x) as.Date(as.character(x)) as.Date(x)
1 2011-12-10 16:55:26 2011-12-10 2011-12-10
2 2011-12-10 17:55:26 2011-12-10 2011-12-10
3 2011-12-10 18:55:26 2011-12-10 2011-12-10
4 2011-12-10 19:55:26 2011-12-10 2011-12-11
5 2011-12-10 20:55:26 2011-12-10 2011-12-11
6 2011-12-10 21:55:26 2011-12-10 2011-12-11
7 2011-12-10 22:55:26 2011-12-10 2011-12-11
8 2011-12-10 23:55:26 2011-12-10 2011-12-11
9 2011-12-11 00:55:26 2011-12-11 2011-12-11
10 2011-12-11 01:55:26 2011-12-11 2011-12-11
>
> # as.Date(POSIXlt) works differently from as.Date(POSIXct)
>
> data.frame(as.character(x), as.Date(x), as.Date(as.POSIXlt(x)), check.names=F)
as.character(x) as.Date(x) as.Date(as.POSIXlt(x))
1 2011-12-10 16:55:26 2011-12-10 2011-12-10
2 2011-12-10 17:55:26 2011-12-10 2011-12-10
3 2011-12-10 18:55:26 2011-12-10 2011-12-10
4 2011-12-10 19:55:26 2011-12-11 2011-12-10
5 2011-12-10 20:55:26 2011-12-11 2011-12-10
6 2011-12-10 21:55:26 2011-12-11 2011-12-10
7 2011-12-10 22:55:26 2011-12-11 2011-12-10
8 2011-12-10 23:55:26 2011-12-11 2011-12-10
9 2011-12-11 00:55:26 2011-12-11 2011-12-11
10 2011-12-11 01:55:26 2011-12-11 2011-12-11
>
> # Sensible conversion POSIXct->character->Date.
> as.Date(as.character(x))
[1] "2011-12-10" "2011-12-10" "2011-12-10" "2011-12-10" "2011-12-10"
[6] "2011-12-10" "2011-12-10" "2011-12-10" "2011-12-11" "2011-12-11"
>
> # Can do this correctly multiple ways, direct POSIXct->Date works if
> # we supply tz='EST' to as.Date()
> all.equal(as.Date(as.character(x)), as.Date(x, tz='EST'))
[1] TRUE
> all.equal(as.Date(as.character(x)), as.Date(as.character(x), tz='EST'))
[1] TRUE
>
> (x.Date <- as.Date(as.character(x)))
[1] "2011-12-10" "2011-12-10" "2011-12-10" "2011-12-10" "2011-12-10"
[6] "2011-12-10" "2011-12-10" "2011-12-10" "2011-12-11" "2011-12-11"
>
> # Sensible conversion Date->character->POSIXct
> as.POSIXct(as.character(x.Date))
[1] "2011-12-10 EST" "2011-12-10 EST" "2011-12-10 EST" "2011-12-10 EST"
[5] "2011-12-10 EST" "2011-12-10 EST" "2011-12-10 EST" "2011-12-10 EST"
[9] "2011-12-11 EST" "2011-12-11 EST"
>
> # Not sensible direct conversion Date->POSIXct. (Unless we really want
> # the time in our zone corresponding to when it is midnight in GMT.)
> as.POSIXct(x.Date)
[1] "2011-12-09 19:00:00 EST" "2011-12-09 19:00:00 EST"
[3] "2011-12-09 19:00:00 EST" "2011-12-09 19:00:00 EST"
[5] "2011-12-09 19:00:00 EST" "2011-12-09 19:00:00 EST"
[7] "2011-12-09 19:00:00 EST" "2011-12-09 19:00:00 EST"
[9] "2011-12-10 19:00:00 EST" "2011-12-10 19:00:00 EST"
>
> # Probably sensible direction conversion Date->POSIXlt -- if this stops
> # being sensible, check code in date*.POSIXlt methods
> as.POSIXlt(x.Date)
[1] "2011-12-10 UTC" "2011-12-10 UTC" "2011-12-10 UTC" "2011-12-10 UTC"
[5] "2011-12-10 UTC" "2011-12-10 UTC" "2011-12-10 UTC" "2011-12-10 UTC"
[9] "2011-12-11 UTC" "2011-12-11 UTC"
>
> # 'tz' argument on as.POSIXlt() is ignored for conversion from Date
> attr(as.POSIXlt(x.Date, tz='UTC'), 'tzone')
[1] "UTC"
> attr(as.POSIXlt(x.Date, tz='GMT'), 'tzone')
[1] "UTC"
> attr(as.POSIXlt(x.Date, tz='EST'), 'tzone')
[1] "UTC"
> all.equal(as.POSIXlt(x.Date), as.POSIXlt(x.Date, tz='UTC'))
[1] TRUE
> all.equal(as.POSIXlt(x.Date), as.POSIXlt(x.Date, tz='GMT'))
[1] TRUE
> all.equal(as.POSIXlt(x.Date), as.POSIXlt(x.Date, tz='EST'))
[1] TRUE
>