diff --git a/emscripten-version.txt b/emscripten-version.txt
index 1a90c51133d32..14094f3776cc6 100644
--- a/emscripten-version.txt
+++ b/emscripten-version.txt
@@ -1,2 +1,2 @@
-1.23.2
+1.23.3
 
diff --git a/src/library.js b/src/library.js
index 55c1959c11093..91f8774753e12 100644
--- a/src/library.js
+++ b/src/library.js
@@ -5783,13 +5783,13 @@ LibraryManager.library = {
     }
     return _usleep((seconds * 1e6) + (nanoseconds / 1000));
   },
-  clock_gettime__deps: ['emscripten_get_now','emscripten_get_now_monotonic','$ERRNO_CODES', '__setErrNo'],
+  clock_gettime__deps: ['emscripten_get_now', 'emscripten_get_now_is_monotonic', '$ERRNO_CODES', '__setErrNo'],
   clock_gettime: function(clk_id, tp) {
     // int clock_gettime(clockid_t clk_id, struct timespec *tp);
     var now;
     if (clk_id === {{{ cDefine('CLOCK_REALTIME') }}}) {
       now = Date.now();
-    } else if (clk_id === {{{ cDefine('CLOCK_MONOTONIC') }}} && _emscripten_get_now_monotonic()) {
+    } else if (clk_id === {{{ cDefine('CLOCK_MONOTONIC') }}} && _emscripten_get_now_is_monotonic()) {
       now = _emscripten_get_now();
     } else {
       ___setErrNo(ERRNO_CODES.EINVAL);
@@ -5803,23 +5803,24 @@ LibraryManager.library = {
   clock_settime: function(clk_id, tp) {
     // int clock_settime(clockid_t clk_id, const struct timespec *tp);
     // Nothing.
-    ___setErrNo(ERRNO_CODES.EPERM);
+    ___setErrNo(clk_id === {{{ cDefine('CLOCK_REALTIME') }}} ? ERRNO_CODES.EPERM
+                                                             : ERRNO_CODES.EINVAL);
     return -1;
   },
-  clock_getres__deps: ['emscripten_get_now_res','emscripten_get_now_monotonic','$ERRNO_CODES', '__setErrNo'],
+  clock_getres__deps: ['emscripten_get_now_res', 'emscripten_get_now_is_monotonic', '$ERRNO_CODES', '__setErrNo'],
   clock_getres: function(clk_id, res) {
     // int clock_getres(clockid_t clk_id, struct timespec *res);
     var nsec;
     if (clk_id === {{{ cDefine('CLOCK_REALTIME') }}}) {
-      nsec = 1000 * 1000;
-    } else if (clk_id === {{{ cDefine('CLOCK_MONOTONIC') }}} && _emscripten_get_now_monotonic()) {
-      now = _emscripten_get_now_res();
+      nsec = 1000 * 1000; // educated guess that it's milliseconds
+    } else if (clk_id === {{{ cDefine('CLOCK_MONOTONIC') }}} && _emscripten_get_now_is_monotonic()) {
+      nsec = _emscripten_get_now_res();
     } else {
       ___setErrNo(ERRNO_CODES.EINVAL);
       return -1;
     }
-    {{{ makeSetValue('res', C_STRUCTS.timespec.tv_sec, '1', 'i32') }}};
-    {{{ makeSetValue('res', C_STRUCTS.timespec.tv_nsec, 'nsec', 'i32') }}} // resolution is milliseconds
+    {{{ makeSetValue('res', C_STRUCTS.timespec.tv_sec, '(nsec/1000000000)|0', 'i32') }}};
+    {{{ makeSetValue('res', C_STRUCTS.timespec.tv_nsec, 'nsec', 'i32') }}} // resolution is nanoseconds
     return 0;
   },
 
@@ -8502,11 +8503,12 @@ LibraryManager.library = {
     }
   },
 
-  emscripten_get_now_monotonic__deps: ['emscripten_get_now'],
-  emscripten_get_now_monotonic: function() {
+  emscripten_get_now_is_monotonic__deps: ['emscripten_get_now'],
+  emscripten_get_now_is_monotonic: function() {
     // return whether emscripten_get_now is guaranteed monotonic; the Date.now
     // implementation is not :(
-    return (_emscripten_get_now() == Date.now)|0;
+    return ENVIRONMENT_IS_NODE || (typeof dateNow !== 'undefined') ||
+        (ENVIRONMENT_IS_WEB && window['performance'] && window['performance']['now']);
   },
 
   // Returns [parentFuncArguments, functionName, paramListName]
diff --git a/tests/core/test_posixtime.in b/tests/core/test_posixtime.in
new file mode 100644
index 0000000000000..d07bd61fd4f37
--- /dev/null
+++ b/tests/core/test_posixtime.in
@@ -0,0 +1,47 @@
+#include <time.h>
+#include <errno.h>
+#include <stdio.h>
+
+int main() {
+  clockid_t clocks[] = { CLOCK_REALTIME, CLOCK_MONOTONIC };
+  for (int i = 0; i < (int)(sizeof(clocks)/sizeof(*clocks)); ++i) {
+    printf("%sTests for clockid_t=%d\n-----------------\n",
+           i == 0 ? "" : "\n", clocks[i]);
+    struct timespec ts;
+    int rv = clock_getres(clocks[i], &ts);
+    if (rv)
+      printf("clock_getres failed\n");
+    else if (ts.tv_sec || ts.tv_nsec > 50000000)
+      printf("clock_getres resolution not enough (%ld.%09ld)\n",
+             (long)ts.tv_sec, ts.tv_nsec);
+    else
+      printf("clock_getres resolution OK\n");
+    rv = clock_gettime(clocks[i], &ts);
+    printf(rv ? "clock_gettime failed\n" : "clock_gettime OK\n");
+    errno = 0;
+    if (clock_settime(clocks[i], &ts) == 0)
+      printf("clock_settime should have failed\n");
+    else if (errno == EPERM && clocks[i] == CLOCK_REALTIME)
+      printf("clock_settime failed with EPERM (OK)\n");
+    else if (errno == EINVAL && clocks[i] == CLOCK_MONOTONIC)
+      printf("clock_settime failed with EINVAL (OK)\n");
+    else
+      printf("clock_settime failed with wrong error code\n");
+  }
+  clockid_t bogus = 42;
+  struct timespec ts;
+  printf("\nTests for clockid_t=%d\n-----------------\n", bogus);
+  if (clock_gettime(bogus, &ts) == 0 || errno != EINVAL)
+    printf("clock_gettime should have failed\n");
+  else
+    printf("clock_gettime failed with EINVAL (OK)\n");
+  if (clock_getres(bogus, &ts) == 0 || errno != EINVAL)
+    printf("clock_getres should have failed\n");
+  else
+    printf("clock_getres failed with EINVAL (OK)\n");
+  if (clock_settime(bogus, &ts) == 0 || errno != EINVAL)
+    printf("clock_settime should have failed\n");
+  else
+    printf("clock_settime failed with EINVAL (OK)\n");
+  return 0;
+}
diff --git a/tests/core/test_posixtime.out b/tests/core/test_posixtime.out
new file mode 100644
index 0000000000000..84417e293820a
--- /dev/null
+++ b/tests/core/test_posixtime.out
@@ -0,0 +1,17 @@
+Tests for clockid_t=0
+-----------------
+clock_getres resolution OK
+clock_gettime OK
+clock_settime failed with EPERM (OK)
+
+Tests for clockid_t=1
+-----------------
+clock_getres resolution OK
+clock_gettime OK
+clock_settime failed with EINVAL (OK)
+
+Tests for clockid_t=42
+-----------------
+clock_gettime failed with EINVAL (OK)
+clock_getres failed with EINVAL (OK)
+clock_settime failed with EINVAL (OK)
diff --git a/tests/test_core.py b/tests/test_core.py
index c68b040e90727..7d37a09ab3ff1 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -4629,6 +4629,12 @@ def test_unistd_misc(self):
       Building.COMPILER_TEST_OPTS += ['-D' + fs]
       self.do_run(src, expected, js_engines=[NODE_JS])
 
+  def test_posixtime(self):
+    test_path = path_from_root('tests', 'core', 'test_posixtime')
+    src, output = (test_path + s for s in ('.in', '.out'))
+
+    self.do_run_from_file(src, output)
+
   def test_uname(self):
     test_path = path_from_root('tests', 'core', 'test_uname')
     src, output = (test_path + s for s in ('.in', '.out'))
diff --git a/tests/time/src.c b/tests/time/src.c
index 61faedfe39312..cbc1a20728255 100644
--- a/tests/time/src.c
+++ b/tests/time/src.c
@@ -67,7 +67,7 @@ int main() {
   time_t t4 = 0;
   time(&t4);
   timespec ts;
-  assert(clock_gettime(0, &ts) == 0);
+  assert(clock_gettime(CLOCK_REALTIME, &ts) == 0);
   assert(abs(ts.tv_sec - t4) <= 2);
   printf("time: %d\n", t4 > 1309635200ll && t4 < 1893362400ll);