If the ms rounded is > 1s, carry that to the second field.

ops/_private/timeconv.py

@@ -32,7 +32,8 @@ def parse_rfc3339(s: str) -> datetime.datetime:
     that Go's encoding/json package produces for time.Time values.
 
     Unfortunately we can't use datetime.fromisoformat(), as that does not
-    support more than 6 digits for the fractional second, nor the 'Z' for UTC.
+    support more than 6 digits for the fractional second, nor the 'Z' for UTC,
+    in Python 3.8 (Python 3.11+ has the required functionality).
     """
     match = _TIMESTAMP_RE.match(s)
     if not match:
@@ -50,6 +51,8 @@ def parse_rfc3339(s: str) -> datetime.datetime:
         tz = datetime.timezone(tz_delta if sign == '+' else -tz_delta)
 
     microsecond = round(float(sfrac or '0') * 1000000)
+    carry, microsecond = divmod(microsecond, 1000000)
+    ss = int(ss) + carry
 
-    return datetime.datetime(int(y), int(m), int(d), int(hh), int(mm), int(ss),
+    return datetime.datetime(int(y), int(m), int(d), int(hh), int(mm), ss,
                              microsecond=microsecond, tzinfo=tz)

test/test_private.py

@@ -72,6 +72,9 @@ def test_parse_rfc3339(self):
         self.assertEqual(timeconv.parse_rfc3339('2020-12-25T13:45:50.123456789+00:00'),
                          datetime.datetime(2020, 12, 25, 13, 45, 50, 123457, tzinfo=utc))
 
+        self.assertEqual(timeconv.parse_rfc3339('2006-08-28T13:20:00.99999999Z'),
+                         datetime.datetime(2006, 8, 28, 13, 20, 1, 0, tzinfo=utc))
+
         tzinfo = datetime.timezone(datetime.timedelta(hours=-11, minutes=-30))
         self.assertEqual(timeconv.parse_rfc3339('2020-12-25T13:45:50.123456789-11:30'),
                          datetime.datetime(2020, 12, 25, 13, 45, 50, 123457, tzinfo=tzinfo))