From 5553c225a4e3b17a1dfb3291faf36108a1422b8b Mon Sep 17 00:00:00 2001
From: Aakriti Kushwaha <aakriti.kushwaha.ak@gmail.com>
Date: Thu, 29 Jan 2026 17:23:07 +0530
Subject: [PATCH] test: fwrite POSIXct should preserve ISO-8601 datetime

---
 inst/tests/fwrite-datetime.Rraw.R | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)
 create mode 100644 inst/tests/fwrite-datetime.Rraw.R

diff --git a/inst/tests/fwrite-datetime.Rraw.R b/inst/tests/fwrite-datetime.Rraw.R
new file mode 100644
index 000000000..c04fa3d31
--- /dev/null
+++ b/inst/tests/fwrite-datetime.Rraw.R
@@ -0,0 +1,30 @@
+test(###
+  #"fwrite: POSIXct should be written as ISO-8601, not numeric seconds",
+  {
+    oldtz = Sys.getenv("TZ", unset = NA)
+    on.exit({
+      if (is.na(oldtz)) Sys.unsetenv("TZ") else Sys.setenv(TZ = oldtz)
+    }, add = TRUE)
+    
+    Sys.setenv(TZ = "UTC")
+    
+    DT = data.table(
+      x = seq(
+        as.POSIXct("1970-01-01", tz = "UTC"),
+        by = "1 sec",
+        length.out = 3
+      )
+    )
+    
+    tmp = tempfile()
+    fwrite(DT, tmp)
+    
+    out = readLines(tmp)
+    
+    stopifnot(
+      out[1L] == "x",
+      out[2L] == "1970-01-01T00:00:00Z",
+      out[3L] == "1970-01-01T00:00:01Z",
+      out[4L] == "1970-01-01T00:00:02Z"
+    )
+  })
\ No newline at end of file