diff --git a/ops/hookcmds/_utils.py b/ops/hookcmds/_utils.py
index 7a85f9de6..912322cc1 100644
--- a/ops/hookcmds/_utils.py
+++ b/ops/hookcmds/_utils.py
@@ -17,6 +17,8 @@
 import datetime
 import subprocess
 
+from .._private import timeconv
+
 
 class Error(Exception):
     """Raised when a hook command exits with a non-zero code."""
@@ -56,8 +58,11 @@ def run(
 
 def datetime_from_iso(dt: str) -> datetime.datetime:
     """Converts a Juju-specific ISO 8601 string to a datetime object."""
-    # Older versions of Python cannot handle the 'Z'.
-    return datetime.datetime.fromisoformat(dt.replace('Z', '+00:00'))
+    # parse_rfc3339 handles arbitrary precision fractional seconds, but requires a timezone.
+    # If no timezone is present, assume UTC (add 'Z').
+    if not dt.endswith('Z') and not ('+' in dt[-6:] or '-' in dt[-6:]):
+        dt = dt + 'Z'
+    return timeconv.parse_rfc3339(dt)
 
 
 def datetime_to_iso(dt: datetime.datetime) -> str:
diff --git a/test/test_hookcmds.py b/test/test_hookcmds.py
index 6f0ea9885..4a48eabae 100644
--- a/test/test_hookcmds.py
+++ b/test/test_hookcmds.py
@@ -897,3 +897,43 @@ def test_storage_list_named(run: Run):
     run.handle(['storage-list', '--format=json', 'stor'], stdout='["stor/1", "stor/2"]')
     result = hookcmds.storage_list('stor')
     assert result == ['stor/1', 'stor/2']
+
+
+@pytest.mark.parametrize(
+    'timestamp,expected',
+    [
+        # Juju 3.6 format (no fractional seconds)
+        (
+            '2026-01-05T23:28:38Z',
+            datetime.datetime(2026, 1, 5, 23, 28, 38, tzinfo=datetime.timezone.utc),
+        ),
+        # Juju 4.0 format (8 digits)
+        (
+            '2026-01-05T23:34:25.50029526Z',
+            datetime.datetime(2026, 1, 5, 23, 34, 25, 500295, tzinfo=datetime.timezone.utc),
+        ),
+        # 5 digits (from issue)
+        (
+            '2026-04-10T18:34:45.65844+00:00',
+            datetime.datetime(2026, 4, 10, 18, 34, 45, 658440, tzinfo=datetime.timezone.utc),
+        ),
+        # Edge case: 1 digit
+        (
+            '2026-01-05T23:34:25.1Z',
+            datetime.datetime(2026, 1, 5, 23, 34, 25, 100000, tzinfo=datetime.timezone.utc),
+        ),
+        # Edge case: 9 digits (nanosecond precision)
+        (
+            '2026-01-05T23:34:25.123456789Z',
+            datetime.datetime(2026, 1, 5, 23, 34, 25, 123457, tzinfo=datetime.timezone.utc),
+        ),
+        # No timezone (assumes UTC)
+        (
+            '2025-08-28T13:20:00',
+            datetime.datetime(2025, 8, 28, 13, 20, 0, tzinfo=datetime.timezone.utc),
+        ),
+    ],
+)
+def test_datetime_from_iso(timestamp: str, expected: datetime.datetime):
+    result = hookcmds._utils.datetime_from_iso(timestamp)
+    assert result == expected