diff --git a/R/fwrite.R b/R/fwrite.R
index 5d91b4e34..c302ebf32 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -121,6 +121,16 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
       x
     })
   }
+  
+  # ensure POSIXct columns are double for proper ISO formatting
+  x <- lapply(x, function(col) {
+    if (inherits(col, "POSIXt")) {
+      # coerce integer-backed to double
+      storage.mode(col) <- "double"
+    }
+    col
+  })
+  
   .Call(CfwriteR, x, file, sep, sep2, eol, na, dec, quote, qmethod=="escape", append,
         row.names, col.names, logical01, scipen, dateTimeAs, buffMB, nThread,
         showProgress, is_gzip, compressLevel, bom, yaml, verbose, encoding, forceDecimal)
diff --git a/inst/tests/fwrite-datetime.Rraw.R b/inst/tests/fwrite-datetime.Rraw.R
new file mode 100644
index 000000000..38d840438
--- /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
diff --git a/inst/tests/tests.Rraw b/inst/tests/tests.Rraw
index 1c7ab6837..4c0c4fc8e 100644
--- a/inst/tests/tests.Rraw
+++ b/inst/tests/tests.Rraw
@@ -21509,3 +21509,4 @@ setdroplevels(x)
 setdroplevels(y)
 test(2364.2, levels(x$a), levels(y$a))
 rm(x, y)
+