Merge pull request pytroll#2676 from pnuu/seviri-nanosecond-times

satpy/readers/seviri_base.py

@@ -475,8 +475,10 @@ def get_cds_time(days, msecs):
         days = np.array([days], dtype="int64")
         msecs = np.array([msecs], dtype="int64")
 
+    # use nanosecond precision to silence warning from XArray
+    nsecs = 1000000 * msecs.astype("timedelta64[ns]")
     time = np.datetime64("1958-01-01").astype("datetime64[ms]") + \
-        days.astype("timedelta64[D]") + msecs.astype("timedelta64[ms]")
+        days.astype("timedelta64[D]") + nsecs
     time[time == np.datetime64("1958-01-01 00:00")] = np.datetime64("NaT")
 
     if len(time) == 1:

satpy/tests/reader_tests/test_seviri_base.py

@@ -74,23 +74,28 @@ def test_chebyshev(self):
         exp = chebyshev4(coefs, time, domain)
         np.testing.assert_allclose(res, exp)
 
-    def test_get_cds_time(self):
-        """Test the get_cds_time function."""
-        # Scalar
+    def test_get_cds_time_scalar(self):
+        """Test the get_cds_time function for scalar inputs."""
         assert get_cds_time(days=21246, msecs=12 * 3600 * 1000) == np.datetime64("2016-03-03 12:00")
 
-        # Array
+    def test_get_cds_time_array(self):
+        """Test the get_cds_time function for array inputs."""
         days = np.array([21246, 21247, 21248])
         msecs = np.array([12*3600*1000, 13*3600*1000 + 1, 14*3600*1000 + 2])
         expected = np.array([np.datetime64("2016-03-03 12:00:00.000"),
                              np.datetime64("2016-03-04 13:00:00.001"),
                              np.datetime64("2016-03-05 14:00:00.002")])
-        np.testing.assert_equal(get_cds_time(days=days, msecs=msecs), expected)
+        res = get_cds_time(days=days, msecs=msecs)
+        np.testing.assert_equal(res, expected)
 
+    def test_get_cds_time_nanoseconds(self):
+        """Test the get_cds_time function for having nanosecond precision."""
         days = 21246
-        msecs = 12*3600*1000
+        msecs = 12 * 3600 * 1000
         expected = np.datetime64("2016-03-03 12:00:00.000")
-        np.testing.assert_equal(get_cds_time(days=days, msecs=msecs), expected)
+        res = get_cds_time(days=days, msecs=msecs)
+        np.testing.assert_equal(res, expected)
+        assert res.dtype == np.dtype("datetime64[ns]")
 
     def test_pad_data_horizontally_bad_shape(self):
         """Test the error handling for the horizontal hrv padding."""