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 <liam.bai...@liamdbailey.com>
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",
#> "POSIXt"), tzone = "UTC")
```
We noticed that this occurs because the tz argument is not passed to
`.Internal(Date2POSIXlt())` inside `as.POSIXlt.Date()`.
Reading through the documentation for `as.POSIX*` we can see that this
behaviour is described:
> “Dates without times are treated as being at midnight UTC.”
In this case, if we want to convert a Date object to POSIX* and specify
a (non-UTC) timezone would the best strategy be to first coerce our Date
object to character? Alternatively, `lubridate::as_datetime()` does seem to
recognise the tz argument and convert a Date object to POSIX* with non-UTC
time zone (see second example below). But it would be nice to know if there
are subtle differences between these two approaches that we should be aware
of.
``` r
foo <- as.Date(33874, origin = "1899-12-30")
dput(foo)
#> structure(8305, class = "Date")
#Convert to POSIXct specifying UTC timezone
bar <- lubridate::as_datetime(foo, tz = "UTC")
dput(as.POSIXlt(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 POSIXct specifying Europe/Berlin
bar <- lubridate::as_datetime(foo, tz = "Europe/Berlin")
dput(as.POSIXlt(bar))
#> structure(list(sec = 0, min = 0L, hour = 0L, mday = 27L, mon = 8L,
#> year = 92L, wday = 0L, yday = 270L, isdst = 1L, zone = "CEST",
#> gmtoff = 7200L), class = c("POSIXlt", "POSIXt"), tzone =
c("Europe/Berlin",
#> "CET", "CEST"))
```
Thanks again for all your help.
Alex & Liam
On 10 Oct 2022, at 6:40 pm, Hadley Wickham <h.wick...@gmail.com> wrote:
On Sun, Oct 9, 2022 at 9:31 PM Jeff Newmiller <jdnew...@dcn.davis.ca.us>
wrote:
... which is why tidyverse functions and Python datetime handling irk
me so much.
Is tidyverse time handling intrinsically broken? They have a standard
practice of reading time as UTC and then using force_tz to fix the
"mistake". Same as Python.
Can you point to any docs that lead you to this conclusion so we can
get them fixed? I strongly encourage people to parse date-times in the
correct time zone; this is why lubridate::ymd_hms() and friends have a
tz argument.
Hadley
--
http://hadley.nz
