From 8d2ed1f041a8132807e135e156ef67a7cb32e8d1 Mon Sep 17 00:00:00 2001
From: Michael Chirico <michael.chirico@grabtaxi.com>
Date: Wed, 4 Dec 2019 15:01:50 +0800
Subject: [PATCH 1/2] default to tz="" for IDateTime POSIX methods

---
 NEWS.md               |  2 ++
 R/IDateTime.R         |  4 ++--
 inst/tests/tests.Rraw | 10 ++++++++++
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/NEWS.md b/NEWS.md
index 4a0033fcfa..4140418b39 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -10,6 +10,8 @@
 
 ## BUG FIXES
 
+1. `as.IDate` and `as.ITime` methods for `POSIXct` input wrongly assigned `NULL` timezone to `UTC` (actually it's the session's default timezone), [#4085](https://github.com/Rdatatable/data.table/issues/4085).
+
 ## NOTES
 
 1. Links in the manual were creating warnings when installing HTML, [#4000](https://github.com/Rdatatable/data.table/issues/4000). Thanks to Morgan Jacob.
diff --git a/R/IDateTime.R b/R/IDateTime.R
index c719b20bb9..265d57ffb3 100644
--- a/R/IDateTime.R
+++ b/R/IDateTime.R
@@ -33,7 +33,7 @@ as.IDate.Date = function(x, ...) {
 }
 
 as.IDate.POSIXct = function(x, tz = attr(x, "tzone", exact=TRUE), ...) {
-  if (is.null(tz)) tz = "UTC"
+  if (is.null(tz)) tz=''
   if (tz %chin% c("UTC", "GMT")) {
     (setattr(as.integer(as.numeric(x) %/% 86400L), "class", c("IDate", "Date")))  # %/% returns new object so can use setattr() on it; wrap with () to return visibly
   } else
@@ -138,7 +138,7 @@ as.ITime.default = function(x, ...) {
 }
 
 as.ITime.POSIXct = function(x, tz = attr(x, "tzone", exact=TRUE), ...) {
-  if (is.null(tz)) tz = "UTC"
+  if (is.null(tz)) tz=''
   if (tz %chin% c("UTC", "GMT")) as.ITime(unclass(x), ...)
   else as.ITime(as.POSIXlt(x, tz = tz, ...), ...)
 }
diff --git a/inst/tests/tests.Rraw b/inst/tests/tests.Rraw
index f10fe26ce1..dedd5d551f 100644
--- a/inst/tests/tests.Rraw
+++ b/inst/tests/tests.Rraw
@@ -16381,6 +16381,16 @@ test(2121.5, DT[, .(.N=.N), by=a], data.table(a=c(1,2), .N=2:1)) # user supplied
 ## { ending in NULL should retain that NULL, #4061
 test(2121.6, DT[ , {.(a, b=b+1); NULL}], NULL)
 
+# system timezone is not usually UTC, so as.ITime.POSIXct shouldn't assume so
+oldtz=Sys.getenv('TZ')
+Sys.setenv(TZ='Asia/Jakarta') # UTC+7
+t0 = Sys.time()
+test(2122.1, as.integer(unclass(as.ITime(t0)) %/% 3600), as.POSIXlt(t0)$hour)
+d0 = as.POSIXct(trunc(t0, 'day'))
+# base defaults
+test(2122.2, unclass(as.IDate(d0)), as.integer(as.Date(d0, tz='')))
+Sys.setenv(TZ=oldtz)
+
 
 ###################################
 #  Add new tests above this line  #

From 374fa8d3368e36b0526b24e89ba200ef673d6636 Mon Sep 17 00:00:00 2001
From: Michael Chirico <michael.chirico@grabtaxi.com>
Date: Wed, 4 Dec 2019 15:05:45 +0800
Subject: [PATCH 2/2] simplify tests

---
 inst/tests/tests.Rraw | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/inst/tests/tests.Rraw b/inst/tests/tests.Rraw
index dedd5d551f..6660052f02 100644
--- a/inst/tests/tests.Rraw
+++ b/inst/tests/tests.Rraw
@@ -16384,11 +16384,9 @@ test(2121.6, DT[ , {.(a, b=b+1); NULL}], NULL)
 # system timezone is not usually UTC, so as.ITime.POSIXct shouldn't assume so
 oldtz=Sys.getenv('TZ')
 Sys.setenv(TZ='Asia/Jakarta') # UTC+7
-t0 = Sys.time()
-test(2122.1, as.integer(unclass(as.ITime(t0)) %/% 3600), as.POSIXlt(t0)$hour)
-d0 = as.POSIXct(trunc(t0, 'day'))
-# base defaults
-test(2122.2, unclass(as.IDate(d0)), as.integer(as.Date(d0, tz='')))
+t0 = as.POSIXct('2019-10-01')
+test(2122.1, format(as.ITime(t0)), '00:00:00')
+test(2122.2, format(as.IDate(t0)), '2019-10-01')
 Sys.setenv(TZ=oldtz)