diff --git a/lib/common/tests/iso8601/pcmk__time_format_hr_test.c b/lib/common/tests/iso8601/pcmk__time_format_hr_test.c
index 0dbcfef71d..41cee602ea 100644
--- a/lib/common/tests/iso8601/pcmk__time_format_hr_test.c
+++ b/lib/common/tests/iso8601/pcmk__time_format_hr_test.c
@@ -1,119 +1,144 @@
 /*
  * Copyright 2024-2025 the Pacemaker project contributors
  *
  * The version control history for this file may have further details.
  *
  * This source code is licensed under the GNU General Public License version 2
  * or later (GPLv2+) WITHOUT ANY WARRANTY.
  */
 
 #include <crm_internal.h>
 
 #include <stdio.h>  // NULL
 
 #include <crm/common/iso8601_internal.h>
 #include <crm/common/unittest_internal.h>
 
 #define TEST_TIME pcmk__time_hr_new("2024-01-02 03:04:05 +00:00")
 
 /*!
  * \internal
  * \brief Assert that pcmk__time_format_hr() produced expected result
  *
  * \param[in] format     Time format string
  * \param[in] expected   Assertion succeeds if result matches this
  * \param[in] alternate  If this is not NULL, assertion may also succeed if
  *                       result matches this
  * \param[in] usec       Microseconds component of the reference time
  *
  * \note This allows two possible results because different strftime()
  *       implementations handle certain format syntax differently.
  */
 static void
 assert_hr_format(const char *format, const char *expected,
                  const char *alternate, int usec)
 {
     pcmk__time_hr_t *hr = TEST_TIME;
     char *result = NULL;
 
     hr->useconds = usec;
     result = pcmk__time_format_hr(format, hr);
     pcmk__time_hr_free(hr);
 
     if (expected == NULL) {
         assert_null(result);
         return;
     }
 
     assert_non_null(result);
 
     if (alternate == NULL) {
         assert_string_equal(result, expected);
     } else {
         assert_true((strcmp(result, expected) == 0)
                     || (strcmp(result, alternate) == 0));
     }
 
     free(result);
 }
 
 static void
 null_format(void **state)
 {
     assert_null(pcmk__time_format_hr(NULL, NULL));
     assert_hr_format(NULL, NULL, NULL, 0); // for pcmk__time_format_hr(NULL, hr)
 }
 
 static void
 no_specifiers(void **state)
 {
     assert_hr_format("no specifiers", "no specifiers", NULL, 0);
     assert_hr_format("this has a literal % in it",
                      "this has a literal % in it",
                      // *BSD strftime() strips single %
                      "this has a literal  in it", 0);
     assert_hr_format("this has a literal %01 in it",
                      "this has a literal %01 in it",
                      // *BSD strftime() strips %0
                      "this has a literal 1 in it", 0);
     assert_hr_format("%2 this starts and ends with %",
                      "%2 this starts and ends with %",
                      // *BSD strftime() strips % in front of nonzero number
                      "2 this starts and ends with %", 0);
 
     /* strftime() treats % with a number (and no specifier) as a literal string
      * to be formatted with a field width (undocumented and probably a bug ...)
      */
     assert_hr_format("this ends with %10", "this ends with        %10",
                      // *BSD strftime() strips % in front of nonzero number
                      "this ends with 10", 0);
 }
 
 static void
 without_nano(void **state)
 {
     assert_hr_format("%H:%M %a %b %d", "03:04 Tue Jan 02", NULL, 0);
     assert_hr_format("%H:%M:%S", "03:04:05", NULL, 0);
     assert_hr_format("The time is %H:%M right now",
                      "The time is 03:04 right now", NULL, 0);
     assert_hr_format("%3S seconds", "005 seconds",
                      // *BSD strftime() doesn't support field widths
                      "3S seconds", 0);
 
     // strftime() treats %% as a literal %
     assert_hr_format("%%H %%N", "%H %N", NULL, 0);
 }
 
 static void
 with_nano(void **state)
 {
+    int usec = 123456;
+
+    // Display time with no fractional seconds component
     assert_hr_format("%H:%M:%S.%06N", "03:04:05.000000", NULL, 0);
+    assert_hr_format("%H:%M:%S.%03N", "03:04:05.000", NULL, 0);
+    assert_hr_format("%H:%M:%S.%00N", "03:04:05.", NULL, 0);
+    assert_hr_format("%H:%M:%S.%0N", "03:04:05.", NULL, 0);
     assert_hr_format("The time is %H:%M:%S.%06N right NOW",
                      "The time is 03:04:05.000000 right NOW", NULL, 0);
+
+    // Display at appropriate resolution by truncating toward zero
+    assert_hr_format("%H:%M:%S.%06N", "03:04:05.123456", NULL, usec);
+    assert_hr_format("%H:%M:%S.%05N", "03:04:05.12345", NULL, usec);
+    assert_hr_format("%H:%M:%S.%04N", "03:04:05.1234", NULL, usec);
+    assert_hr_format("%H:%M:%S.%03N", "03:04:05.123", NULL, usec);
+    assert_hr_format("%H:%M:%S.%02N", "03:04:05.12", NULL, usec);
+    assert_hr_format("%H:%M:%S.%01N", "03:04:05.1", NULL, usec);
+    assert_hr_format("%H:%M:%S.%00N", "03:04:05.", NULL, usec);
+
+    // Same as above, but with correct zero-padding
+    usec = 789;
+    assert_hr_format("%H:%M:%S.%06N", "03:04:05.000789", NULL, usec);
+    assert_hr_format("%H:%M:%S.%05N", "03:04:05.00078", NULL, usec);
+    assert_hr_format("%H:%M:%S.%04N", "03:04:05.0007", NULL, usec);
+    assert_hr_format("%H:%M:%S.%03N", "03:04:05.000", NULL, usec);
+    assert_hr_format("%H:%M:%S.%02N", "03:04:05.00", NULL, usec);
+    assert_hr_format("%H:%M:%S.%01N", "03:04:05.0", NULL, usec);
+    assert_hr_format("%H:%M:%S.%00N", "03:04:05.", NULL, usec);
 }
 
 PCMK__UNIT_TEST(NULL, NULL,
                 cmocka_unit_test(null_format),
                 cmocka_unit_test(no_specifiers),
                 cmocka_unit_test(without_nano),
                 cmocka_unit_test(with_nano))