diff --git a/NEWS.md b/NEWS.md
index 021ddbbb6..47c915141 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -50,6 +50,8 @@
 
 9. `fread()` no longer replaces a literal header column name `"NA"` with an auto-generated `Vn` name when `na.strings` includes `"NA"`, [#5124](https://github.com/Rdatatable/data.table/issues/5124). Data rows still continue to parse `"NA"` as missing. Thanks @Mashin6 for the report and @shrektan for the fix.
 
+10. `fwrite` with a `POSIXct` column backed by integer storage (e.g. created via `seq()`) would write epoch integers instead of ISO datetime strings, [#3535](https://github.com/Rdatatable/data.table/issues/3535). Thanks to @alistaire47 for reporting and @tjc234 and @rmwood0908 for fixing.
+
 ### Notes
 
 1. {data.table} now depends on R 3.5.0 (2018).
diff --git a/inst/tests/tests.Rraw b/inst/tests/tests.Rraw
index 443487c6a..a880c3065 100644
--- a/inst/tests/tests.Rraw
+++ b/inst/tests/tests.Rraw
@@ -21585,3 +21585,20 @@ close(con)
 file.create(f <- tempfile())
 test(2367.6, fread(file(f)), data.table(), warning="Connection has size 0.")
 unlink(f)
+# fwrite ignores dateTimeAs for integer-backed POSIXct created by seq() #3535
+oldtz = Sys.getenv('TZ')
+Sys.setenv(TZ = 'UTC')
+tmp <- tempfile()
+on.exit(Sys.setenv(TZ = oldtz), add = TRUE)
+DT <- data.table(x = seq(as.POSIXct('1970-01-01'), by = '1 sec', length = 6))
+fwrite(DT, tmp)
+test(2368.1, capture.output(cat(readLines(tmp), sep = '\n')), c(
+  "x",
+  "1970-01-01T00:00:00Z",
+  "1970-01-01T00:00:01Z",
+  "1970-01-01T00:00:02Z",
+  "1970-01-01T00:00:03Z",
+  "1970-01-01T00:00:04Z",
+  "1970-01-01T00:00:05Z"
+))
+rm(DT, tmp, oldtz)
diff --git a/src/fwrite.c b/src/fwrite.c
index 670936423..e8f181c1d 100644
--- a/src/fwrite.c
+++ b/src/fwrite.c
@@ -480,6 +480,16 @@ void writePOSIXct(const void *col, int64_t row, char **pch)
   *pch = ch;
 }
 
+void writePOSIXctInt(const void *col, int64_t row, char **pch)
+{
+  // integer-backed POSIXct
+  // before formatting, since writePOSIXct expects double storage
+  int32_t xi = ((const int32_t*)col)[row];
+  double x = (xi == INT32_MIN) ? NAN : (double)xi;
+  const void *tmp = &x;
+  writePOSIXct(tmp, 0, pch);
+}
+
 // # nocov start. Covered in other.Rraw test 22, not the main suite.
 void writeNanotime(const void *col, int64_t row, char **pch)
 {
diff --git a/src/fwrite.h b/src/fwrite.h
index 01b795b6f..412510e53 100644
--- a/src/fwrite.h
+++ b/src/fwrite.h
@@ -23,6 +23,7 @@ writer_fun_t writeITime;
 writer_fun_t writeDateInt32;
 writer_fun_t writeDateFloat64;
 writer_fun_t writePOSIXct;
+writer_fun_t writePOSIXctInt;
 writer_fun_t writeNanotime;
 writer_fun_t writeString;
 writer_fun_t writeCategString;
@@ -42,6 +43,7 @@ typedef enum {   // same order as fun[] above
   WF_DateInt32,
   WF_DateFloat64,
   WF_POSIXct,
+  WF_POSIXctInt,
   WF_Nanotime,
   WF_String,
   WF_CategString,
@@ -60,6 +62,7 @@ static const int writerMaxLen[] = {  // same order as fun[] and WFs above; max f
   16, //&writeDateInt32
   16, //&writeDateFloat64
   32, //&writePOSIXct
+  32, //&writePOSIXctInt
   48, //&writeNanotime
   0,  //&writeString
   0,  //&writeCategString
diff --git a/src/fwriteR.c b/src/fwriteR.c
index b2d631fb3..f10b26aaa 100644
--- a/src/fwriteR.c
+++ b/src/fwriteR.c
@@ -65,6 +65,7 @@ writer_fun_t *funs[] = {
   &writeDateInt32,
   &writeDateFloat64,
   &writePOSIXct,
+  &writePOSIXctInt,
   &writeNanotime,
   &writeString,
   &writeCategString,
@@ -128,6 +129,7 @@ static int32_t whichWriter(SEXP column) {
     if (dateTimeAs == DATETIMEAS_EPOCH)    return WF_Int32;
     if (INHERITS(column, char_ITime))      return WF_ITime;
     if (INHERITS(column, char_Date))       return WF_DateInt32;
+    if (INHERITS(column, char_POSIXct))    return WF_POSIXctInt;
     return WF_Int32;
   case REALSXP:
     if (INHERITS(column, char_nanotime)