[R-pkg-devel] Issue handling datetimes: possible differences between computers

[Alexandre Courtiol]

Hi R pkg developers,

We are facing a datetime handling issue which manifests itself in a
package we are working on.

In context, we noticed that reading datetime info from an excel file
resulted in different data depending on the computer we used.

We are aware that timezone and regional settings are general sources
of troubles, but the code we are using was trying to circumvent this.
We went only as far as figuring out that the issue happens when
converting a POSIXlt into a POSIXct.

Please find below, a minimal reproducible example where `foo` is
converted to `bar` on two different computers.
`foo` is a POSIXlt with a defined time zone and upon conversion to a
POSIXct, despite using a set time zone, we end up with `bar` being
different on Linux and on a Windows machine.

We noticed that the difference emerges from the system call
`.Internal(as.POSIXct())` within `as.POSIXct.POSIXlt()`.
We also noticed that the internal function in R actually calls
getenv("TZ") within C, which is probably what explains where the
difference comes from.

Such a behaviour is probably expected and not a bug, but what would be
the strategy to convert a POSIXlt into a POSIXct that would not be
machine dependent?

We finally noticed that depending on the datetime used as a starting
point and on the time zone used when calling `as.POSIXct()`, we
sometimes have a difference between computers and sometimes not...
which adds to our puzzlement.

Many thanks.
Alex & Liam


``` r
## On Linux
foo <- structure(list(sec = 0, min = 0L, hour = 0L, mday = 1L, mon =
9L, year = 121L, wday = 5L, yday = 273L, isdst = 0L),
                 class = c("POSIXlt", "POSIXt"), tzone = "UTC")

bar <- as.POSIXct(foo, tz = "Europe/Berlin")

bar
#> [1] "2021-10-01 01:00:00 CEST"

dput(bar)
#> structure(1633042800, class = c("POSIXct", "POSIXt"), tzone =
"Europe/Berlin")
```

``` r
## On Windows
foo <- structure(list(sec = 0, min = 0L, hour = 0L, mday = 1L, mon =
9L, year = 121L, wday = 5L, yday = 273L, isdst = 0L),
                 class = c("POSIXlt", "POSIXt"), tzone = "UTC")

bar <- as.POSIXct(foo, tz = "Europe/Berlin")

bar
#> [1] "2021-10-01 CEST"

dput(bar)
structure(1633046400, class = c("POSIXct", "POSIXt"), tzone = "Europe/Berlin")
```

-- 
Alexandre Courtiol, www.datazoogang.de