Fix getting channel time track when using raw timestamps (#254)

nptdms/tdms.py

@@ -637,6 +637,8 @@ def time_track(self, absolute_time=False, accuracy='ns'):
         except KeyError:
             raise KeyError(
                 "Object does not have start time property available.")
+        if isinstance(start_time, TdmsTimestamp):
+            start_time = start_time.as_datetime64(accuracy)
 
         try:
             unit_correction = {
@@ -651,7 +653,7 @@ def time_track(self, absolute_time=False, accuracy='ns'):
         # Because numpy only knows ints as its date datatype,
         # convert to accuracy.
         time_type = "timedelta64[{0}]".format(accuracy)
-        return (np.datetime64(start_time) +
+        return (start_time +
                 (relative_time * unit_correction).astype(time_type))
 
     def as_dataframe(self, time_index=False, absolute_time=False, scaled_data=True):

nptdms/test/test_timestamps.py

@@ -90,6 +90,38 @@ def test_read_raw_timestamp_data():
         np.testing.assert_equal(data.second_fractions, expected_second_fractions)
 
 
+def test_time_track_with_raw_timestamps():
+    """ Test getting a channel's time track when using raw timestamps
+    """
+    test_file = GeneratedFile()
+    second_fractions = 1234567890 * 10 ** 10
+    seconds = 3524551547
+    properties = {
+        "wf_start_time": (0x44, hexlify_value("<Q", second_fractions) + hexlify_value("<q", seconds)),
+        "wf_increment": (10, hexlify_value("<d", 0.1)),
+        "wf_start_offset": (10, hexlify_value("<d", 0.0)),
+    }
+    test_file.add_segment(
+        ("kTocMetaData", "kTocRawData", "kTocNewObjList"),
+        segment_objects_metadata(
+            channel_metadata("/'group'/'channel1'", 3, 4, properties),
+        ),
+        "00 00 00 00" "01 00 00 00" "02 00 00 00" "03 00 00 00"
+    )
+    expected_times = np.array([
+        np.datetime64('2015-09-08T10:05:47.669260594', 'ns'),
+        np.datetime64('2015-09-08T10:05:47.769260594', 'ns'),
+        np.datetime64('2015-09-08T10:05:47.869260594', 'ns'),
+        np.datetime64('2015-09-08T10:05:47.969260594', 'ns'),
+    ])
+
+    with test_file.get_tempfile() as temp_file:
+        tdms_data = TdmsFile.read(temp_file.file, raw_timestamps=True)
+        channel = tdms_data["group"]["channel1"]
+        time_track = channel.time_track(absolute_time=True, accuracy='ns')
+        np.testing.assert_array_equal(time_track, expected_times)
+
+
 def test_read_big_endian_timestamp_data():
     seconds = 3672033330
     second_fractions = 1234567890 * 10 ** 10