diff --git a/lib/common/tests/iso8601/Makefile.am b/lib/common/tests/iso8601/Makefile.am
index 581115a2d1..5135f4706b 100644
--- a/lib/common/tests/iso8601/Makefile.am
+++ b/lib/common/tests/iso8601/Makefile.am
@@ -1,18 +1,19 @@
 #
 # Copyright 2020-2024 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 $(top_srcdir)/mk/tap.mk
 include $(top_srcdir)/mk/unittest.mk
 
 # Add "_test" to the end of all test program names to simplify .gitignore.
 check_PROGRAMS = pcmk__add_time_from_xml_test		\
 		 pcmk__readable_interval_test		\
-		 pcmk__set_time_if_earlier_test
+		 pcmk__set_time_if_earlier_test		\
+		 pcmk__time_format_hr_test
 
 TESTS = $(check_PROGRAMS)
diff --git a/lib/common/tests/iso8601/pcmk__time_format_hr_test.c b/lib/common/tests/iso8601/pcmk__time_format_hr_test.c
new file mode 100644
index 0000000000..c7b821bb2d
--- /dev/null
+++ b/lib/common/tests/iso8601/pcmk__time_format_hr_test.c
@@ -0,0 +1,109 @@
+/*
+ * Copyright 2024 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
+ *
+ * \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)
+{
+    pcmk__time_hr_t *hr = TEST_TIME;
+    char *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); // for pcmk__time_format_hr(NULL, hr)
+}
+
+static void
+no_specifiers(void **state)
+{
+    assert_hr_format("no specifiers", "no specifiers", NULL);
+    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");
+    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");
+    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 %");
+}
+
+static void
+without_nano(void **state)
+{
+    assert_hr_format("%H:%M %a %b %d", "03:04 Tue Jan 02", NULL);
+    assert_hr_format("%H:%M:%S", "03:04:05", NULL);
+    assert_hr_format("The time is %H:%M right now",
+                     "The time is 03:04 right now", NULL);
+    assert_hr_format("%3S seconds", "005 seconds",
+                     // *BSD strftime() doesn't support field widths
+                     "3S seconds");
+
+    // strftime() treats %% as a literal %
+    assert_hr_format("%%H %%N", "%H %N", NULL);
+}
+
+static void
+with_nano(void **state)
+{
+    assert_hr_format("%H:%M:%S.%06N", "03:04:05.000000", NULL);
+    assert_hr_format("The time is %H:%M:%S.%06N right NOW",
+                     "The time is 03:04:05.000000 right NOW", NULL);
+}
+
+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))