diff --git a/NEWS.md b/NEWS.md
index d670627825..c586401d87 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -8,6 +8,8 @@
 
 ## BUG FIXES
 
+1. A NULL timezone on POSIXct was interpreted by `as.IDate` and `as.ITime` as UTC rather than the session's default timezone (`tz=""`) , [#4085](https://github.com/Rdatatable/data.table/issues/4085).
+
 ## NOTES
 
 
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 d287a39856..80f2bc25e1 100644
--- a/inst/tests/tests.Rraw
+++ b/inst/tests/tests.Rraw
@@ -16392,6 +16392,14 @@ test(2122.2, DT, data.table(V3=5:6))
 dt = data.table(SomeNumberA=c(1,1,1),SomeNumberB=c(1,1,1))
 test(2123, dt[, .(.N, TotalA=sum(SomeNumberA), TotalB=sum(SomeNumberB)), by=SomeNumberA], data.table(SomeNumberA=1, N=3L, TotalA=1, TotalB=3))
 
+# system timezone is not usually UTC, so as.ITime.POSIXct shouldn't assume so, #4085
+oldtz=Sys.getenv('TZ')
+Sys.setenv(TZ='Asia/Jakarta') # UTC+7
+t0 = as.POSIXct('2019-10-01')
+test(2124.1, format(as.ITime(t0)), '00:00:00')
+test(2124.2, format(as.IDate(t0)), '2019-10-01')
+Sys.setenv(TZ=oldtz)
+
 
 ###################################
 #  Add new tests above this line  #