> Ben Bolker
> on Mon, 10 Oct 2022 16:59:35 -0400 writes:
> Right now as.POSIXlt.Date() is just
> function (x, ...)
> .Internal(Date2POSIXlt(x))
It has been quite a bit different in R-devel for a little
while. NEWS entries (there are more already, and more coming
on the wide topic)
* The as.POSIXlt() and as.POSIXct() default
methods now do obey their tz argument, also in this case.
* as.POSIXlt() now does apply a tz (timezone) argument, as
does as.POSIXct(); partly suggested by Roland Fuss on the R-devel
mailing list.
and indeed it would have been good had you used (and read) the
R-devel mailing list which is much more appropriate on the
topic of *changing* base R behavior.
> How expensive would it be to throw a warning when '...' is provided by
> the user/discarded ??
> Alternately, perhaps the documentation could be amended, although I'm
> not quite sure what to suggest. (The sentence Liam refers to, "Dates
> without times are treated as being at midnight UTC." is correct but
> terse ...)
> On 2022-10-10 4:50 p.m., Alexandre Courtiol wrote:
>> Hi Simon,
>>
>> Thanks for the clarification.
>>
>> From a naive developer point of view, we were initially baffled that the
>> generic as.POSIXlt() does very different things on a character and on a
>> Date input:
>>
>> as.POSIXlt(as.character(foo), "Europe/Berlin")
>> [1] "1992-09-27 CEST"
>>
>> as.POSIXlt(foo, "Europe/Berlin")
>> [1] "1992-09-27 UTC"
>>
>> Based on what you said, it does make sense: it is only when creating the
>> date/time that we want to include the time zone and that only happens
when
>> we don't already work on a previously created date.
>> That is your subtle but spot-on distinction between "parsing" and
>> "changing" the time zone.
>>
>> Yet, we do find it dangerous that as.POSIXlt.Date() accepts a time zone
but
>> does nothing of it, especially when the help file starts with:
>>
>> Usage
>> as.POSIXlt(x, tz = "", ...)
>>
>> The behaviour is documented, as Liam reported it, but still, we will
almost
>> certainly not be the last one tripping on this (without even adding the
>> additional issue of as.POSIXct() behaving differently across OS).
>>
>> Thanks again,
>>
>> Alex & Liam
>>
>>
>>
>>
>> On Mon, 10 Oct 2022 at 22:13, Simon Urbanek
>> wrote:
>>
>>> Liam,
>>>
>>> I think I have failed to convey my main point in the last e-mail - which
>>> was that you want to parse the date/time in the timezone that you care
>>> about so in your example that would be
>>>
foo <- as.Date(33874, origin = "1899-12-30")
foo
>>> [1] "1992-09-27"
as.POSIXlt(as.character(foo), "Europe/Berlin")
>>> [1] "1992-09-27 CEST"
>>>
>>> I was explicitly saying that you do NOT want to simply change the time
>>> zone on POSIXlt objects as that won't work for reasons I explained -
see my
>>> last e-mail.
>>>
>>> Cheers,
>>> Simon
>>>
>>>
On 11/10/2022, at 6:31 AM, Liam Bailey
>>> wrote:
Hi all,
Thanks Simon for the detailed response, that helps us understand a lot
>>> better what’s going on! However, with your response in mind, we still
>>> encounter some behaviour that we did not expect.
I’ve included another minimum reproducible example below to expand on
>>> the situation. In this example, `foo` is a Date object that we generate
>>> from a numeric input. Following your advice, `bar` is then a POSIXlt
object
>>> where we now explicitly define timezone using argument tz. However, even
>>> though we are explicit about the timezone the POSIXlt that is generated
is
>>> always in UTC. This then leads to the issues outlined by Alexandre
above,
>>> which we now understand are caused by DST.
``` r
#Generate date from numeric
#Not possible to specify tz at this point
foo <- as.Date(33874, origin = "1899-12-30")
dput(foo)
#> structure(8305, class = "Date")
#Convert to POSIXlt specifying UTC timezone
bar <- as.POSIXlt(foo, tz = "UTC")
dput(bar)
#> structure(list(sec = 0, min = 0L, hour = 0L, mday = 27L, mon = 8L,
#> year = 92L, wday = 0L, yday = 270L, isdst = 0L), class =
>>> c("POSIXlt",
#> "POSIXt"), tzone = "UTC")
#Convert to POSIXlt specifying Europe/Berlin.
#Time zone is still UTC
bar <- as.POSIXlt(foo, tz = "Europe/Berlin")
dput(bar)
#> structure(list(sec = 0, min = 0L, hour = 0L, mday = 27L, mon = 8L,
#> year = 92L, wday = 0L, yday = 270L, isdst = 0L), class =
>>> c("POSIXlt",
#>
