diff --git a/tests/unittests/tests-fmt/tests-fmt.c b/tests/unittests/tests-fmt/tests-fmt.c
index c9d4b45d98..e707e8056f 100644
--- a/tests/unittests/tests-fmt/tests-fmt.c
+++ b/tests/unittests/tests-fmt/tests-fmt.c
@@ -931,6 +931,98 @@ static void test_fmt_lpad(void)
     TEST_ASSERT_EQUAL_STRING((char*)string, "xxxx3333");
 }
 
+static void test_fmt_time_iso8601(void)
+{
+    char out[20] = { 0 };
+    const char *expected = "2025-04-08T17:40:02";
+
+    struct tm time = {
+        .tm_year = 2025 - 1900,
+        .tm_mon = 4 - 1,
+        .tm_mday = 8,
+        .tm_hour = 17,
+        .tm_min = 40,
+        .tm_sec = 2
+    };
+
+    TEST_ASSERT_EQUAL_INT(19, fmt_time_tm_iso8601(out, &time, 'T'));
+    TEST_ASSERT_EQUAL_STRING(expected, out);
+    expected = "2025-04-08 17:40:02";
+    TEST_ASSERT_EQUAL_INT(19, fmt_time_tm_iso8601(out, &time, ' '));
+    TEST_ASSERT_EQUAL_STRING(expected, out);
+
+    time.tm_year = -1901;
+    TEST_ASSERT_EQUAL_INT(-EINVAL, fmt_time_tm_iso8601(out, &time, 'T'));
+    time.tm_year = 9999 - 1899;
+    TEST_ASSERT_EQUAL_INT(-EINVAL, fmt_time_tm_iso8601(out, &time, 'T'));
+}
+
+static void test_scn_time_iso8601(void)
+{
+    struct tm time;
+
+    memset(&time, 0, sizeof(time));
+    TEST_ASSERT_EQUAL_INT(19, scn_time_tm_iso8601(&time, "2025-04-08T17:40:02", 'T'));
+    TEST_ASSERT_EQUAL_INT(2025 - 1900, time.tm_year);
+    TEST_ASSERT_EQUAL_INT(4 - 1, time.tm_mon);
+    TEST_ASSERT_EQUAL_INT(8, time.tm_mday);
+    TEST_ASSERT_EQUAL_INT(17, time.tm_hour);
+    TEST_ASSERT_EQUAL_INT(40, time.tm_min);
+    TEST_ASSERT_EQUAL_INT(2, time.tm_sec);
+
+    memset(&time, 0, sizeof(time));
+    TEST_ASSERT_EQUAL_INT(19, scn_time_tm_iso8601(&time, "2025-04-08 17:40:02", ' '));
+    TEST_ASSERT_EQUAL_INT(2025 - 1900, time.tm_year);
+    TEST_ASSERT_EQUAL_INT(4 - 1, time.tm_mon);
+    TEST_ASSERT_EQUAL_INT(8, time.tm_mday);
+    TEST_ASSERT_EQUAL_INT(17, time.tm_hour);
+    TEST_ASSERT_EQUAL_INT(40, time.tm_min);
+    TEST_ASSERT_EQUAL_INT(2, time.tm_sec);
+
+    memset(&time, 0, sizeof(time));
+    TEST_ASSERT_EQUAL_INT(10, scn_time_tm_iso8601(&time, "2025-04-08", 'T'));
+    TEST_ASSERT_EQUAL_INT(2025 - 1900, time.tm_year);
+    TEST_ASSERT_EQUAL_INT(4 - 1, time.tm_mon);
+    TEST_ASSERT_EQUAL_INT(8, time.tm_mday);
+
+    memset(&time, 0, sizeof(time));
+    TEST_ASSERT_EQUAL_INT(19, scn_time_tm_iso8601(&time, "2025-13-08T17:40:02", 'T'));
+    TEST_ASSERT_EQUAL_INT(2025 - 1900, time.tm_year);
+    TEST_ASSERT_EQUAL_INT(13 - 1, time.tm_mon);
+    TEST_ASSERT_EQUAL_INT(8, time.tm_mday);
+    TEST_ASSERT_EQUAL_INT(17, time.tm_hour);
+    TEST_ASSERT_EQUAL_INT(40, time.tm_min);
+    TEST_ASSERT_EQUAL_INT(2, time.tm_sec);
+
+    memset(&time, 0, sizeof(time));
+    TEST_ASSERT_EQUAL_INT(19, scn_time_tm_iso8601(&time, "2025-04-08T17:60:02", 'T'));
+    TEST_ASSERT_EQUAL_INT(2025 - 1900, time.tm_year);
+    TEST_ASSERT_EQUAL_INT(4 - 1, time.tm_mon);
+    TEST_ASSERT_EQUAL_INT(8, time.tm_mday);
+    TEST_ASSERT_EQUAL_INT(17, time.tm_hour);
+    TEST_ASSERT_EQUAL_INT(60, time.tm_min);
+    TEST_ASSERT_EQUAL_INT(2, time.tm_sec);
+
+    memset(&time, 0, sizeof(time));
+    TEST_ASSERT_EQUAL_INT(19, scn_time_tm_iso8601(&time, "1899-04-08T17:40:02", 'T'));
+    TEST_ASSERT_EQUAL_INT(1899 - 1900, time.tm_year);
+    TEST_ASSERT_EQUAL_INT(4 - 1, time.tm_mon);
+    TEST_ASSERT_EQUAL_INT(8, time.tm_mday);
+    TEST_ASSERT_EQUAL_INT(17, time.tm_hour);
+    TEST_ASSERT_EQUAL_INT(40, time.tm_min);
+    TEST_ASSERT_EQUAL_INT(2, time.tm_sec);
+
+    memset(&time, 0, sizeof(time));
+    TEST_ASSERT_EQUAL_INT(-EBADF, scn_time_tm_iso8601(&time, "2025-04-08T", ' '));
+    TEST_ASSERT_EQUAL_INT(-EBADF, scn_time_tm_iso8601(&time, "2025-04-08 17:40:02", 'T'));
+    TEST_ASSERT_EQUAL_INT(-EINVAL, scn_time_tm_iso8601(&time, "2025-XX-08T17:40:02", 'T'));
+    TEST_ASSERT_EQUAL_INT(-EINVAL, scn_time_tm_iso8601(&time, "2025-4-08T17:40:02", 'T'));
+    TEST_ASSERT_EQUAL_INT(-EINVAL, scn_time_tm_iso8601(&time, "2025-04-8T17:40:02", 'T'));
+    TEST_ASSERT_EQUAL_INT(-EINVAL, scn_time_tm_iso8601(&time, "2025-04-08T17:40:2", 'T'));
+    TEST_ASSERT_EQUAL_INT(-EINVAL, scn_time_tm_iso8601(&time, "-2025-04-08T17:40:02", 'T'));
+    TEST_ASSERT_EQUAL_INT(-EINVAL, scn_time_tm_iso8601(&time, "10000-04-08T17:40:02", 'T'));
+}
+
 Test *tests_fmt_tests(void)
 {
     EMB_UNIT_TESTFIXTURES(fixtures) {
@@ -965,6 +1057,8 @@ Test *tests_fmt_tests(void)
         new_TestFixture(test_scn_u32_hex),
         new_TestFixture(test_scn_buf_hex),
         new_TestFixture(test_fmt_lpad),
+        new_TestFixture(test_fmt_time_iso8601),
+        new_TestFixture(test_scn_time_iso8601),
     };
 
     EMB_UNIT_TESTCALLER(fmt_tests, NULL, NULL, fixtures);