diff --git a/satpy/readers/ahi_hsd.py b/satpy/readers/ahi_hsd.py
index 116f5e7d5a..92b568d87a 100644
--- a/satpy/readers/ahi_hsd.py
+++ b/satpy/readers/ahi_hsd.py
@@ -436,6 +436,13 @@ def nominal_end_time(self):
         """Get the nominal end time."""
         return self._modify_observation_time_for_nominal(self.observation_end_time)
 
+    @staticmethod
+    def _is_valid_timeline(timeline):
+        """Check that the `observation_timeline` value is not a fill value."""
+        if int(timeline[:2]) > 23:
+            return False
+        return True
+
     def _modify_observation_time_for_nominal(self, observation_time):
         """Round observation time to a nominal time based on known observation frequency.
 
@@ -450,11 +457,16 @@ def _modify_observation_time_for_nominal(self, observation_time):
 
         """
         timeline = "{:04d}".format(self.basic_info['observation_timeline'][0])
+        if not self._is_valid_timeline(timeline):
+            warnings.warn("Observation timeline is fill value, not rounding observation time.")
+            return observation_time
+
         if self.observation_area == 'FLDK':
             dt = 0
         else:
             observation_frequency_seconds = {'JP': 150, 'R3': 150, 'R4': 30, 'R5': 30}[self.observation_area[:2]]
             dt = observation_frequency_seconds * (int(self.observation_area[2:]) - 1)
+
         return observation_time.replace(
             hour=int(timeline[:2]), minute=int(timeline[2:4]) + dt//60,
             second=dt % 60, microsecond=0)
diff --git a/satpy/tests/reader_tests/test_ahi_hsd.py b/satpy/tests/reader_tests/test_ahi_hsd.py
index 3a2495d5d0..1112ebe5a5 100644
--- a/satpy/tests/reader_tests/test_ahi_hsd.py
+++ b/satpy/tests/reader_tests/test_ahi_hsd.py
@@ -37,11 +37,11 @@
 
 FAKE_BASIC_INFO: InfoDict = {
     'blocklength': 0,
-    'satellite': np.array(['Himawari-8']),
-    'observation_area': np.array(['FLDK']),
-    'observation_start_time': np.array([58413.12523839]),
-    'observation_end_time': np.array([58413.12562439]),
-    'observation_timeline': np.array([300]),
+    'satellite': 'Himawari-8',
+    'observation_area': 'FLDK',
+    'observation_start_time': 58413.12523839,
+    'observation_end_time': 58413.12562439,
+    'observation_timeline': '0300',
 }
 FAKE_DATA_INFO: InfoDict = {
     'blocklength': 50,
@@ -359,6 +359,23 @@ def test_blocklen_error(self, *mocks):
                 fh._check_fpos(fp_, fpos, 0, 'header 1')
                 assert len(w) > 0
 
+    def test_is_valid_time(self):
+        """Test that valid times are correctly identified."""
+        assert AHIHSDFileHandler._is_valid_timeline(FAKE_BASIC_INFO['observation_timeline'])
+        assert not AHIHSDFileHandler._is_valid_timeline('65526')
+
+    def test_time_rounding(self):
+        """Test rounding of the nominal time."""
+        mocker = mock.MagicMock()
+        in_date = datetime(2020, 1, 1, 12, 0, 0)
+
+        with mock.patch('satpy.readers.ahi_hsd.AHIHSDFileHandler._is_valid_timeline', mocker):
+            with _fake_hsd_handler() as fh:
+                mocker.return_value = True
+                assert fh._modify_observation_time_for_nominal(in_date) == datetime(2020, 1, 1, 3, 0, 0)
+                mocker.return_value = False
+                assert fh._modify_observation_time_for_nominal(in_date) == datetime(2020, 1, 1, 12, 0, 0)
+
 
 class TestAHICalibration(unittest.TestCase):
     """Test case for various AHI calibration types."""