with_timezone() ?

[gaborcsardi]

I realize this is somewhat niche, but it can be useful for testing package that deal with timestamps.

The implementation is mostly straightforward, I can submit a PR, I need to write this for the parsedate package, anyway.

[gaborcsardi]

It seems that this would do it, although I still need to test it on older R versions, I am not sure when the cache was introduced:

with_timezone <- function(tzone, expr) {
  ## Need to remove the cache first, and also on exit
  clear <- function() assign(".sys.timezone", NA, envir = baseenv())
  clear()
  on.exit(clear(), add = TRUE)
  with_envvar(c(TZ = tzone), expr)
}

Sys.timezone()
#> [1] "Europe/London"

with_timezone("CET", Sys.timezone())
#> [1] "CET"

[gaborcsardi]

We could probably also just restore the cache on exit, instead of setting it to NA. Just in case:

with_timezone <- function(tzone, expr) {
  old <- get0(".sys.timezone", baseenv(), mode = "character",
              inherits = FALSE, ifnotfound = NA_character_)
  on.exit(assign(".sys.timezone", old, envir = baseenv()), add = TRUE)
  assign(".sys.timezone", NA, envir = baseenv())
  withr::with_envvar(c(TZ = tzone), expr)
}

[gaborcsardi]

Actually, there is something more to it, because this does not work:

with_timezone("US/Pacific", Sys.time())
#> [1] "2019-05-08 10:44:45 BST"

EDIT: no, it is good, just need to print inside:

with_timezone("US/Pacific", print(Sys.time()))
#> [1] "2019-05-08 02:45:43 PDT"

[jimhester]

I think it is definitely worthwhile to have in withr, particularly because it has some fiddly bits with the cache. So a PR would be great!

[gaborcsardi]

OK, I have an implementation in parsedate, so I'll submit a PR soon.

[gaborcsardi]

Do you want to call it with_tz() or with_timezone() or with_time_zone() or sg else?

[jimhester]

I guess with_timezone() to match Sys.timezone().