Fix Kvaser timestamp (#1878)

* get timestamp offset after canBusOn

* update comment

can/interfaces/kvaser/canlib.py

@@ -573,35 +573,39 @@ def __init__(
         )
         canSetBusOutputControl(self._write_handle, can_driver_mode)
 
+        self._is_filtered = False
+        super().__init__(
+            channel=channel,
+            can_filters=can_filters,
+            **kwargs,
+        )
+
+        # activate channel after CAN filters were applied
+        log.debug("Go on bus")
+        if not self.single_handle:
+            canBusOn(self._read_handle)
+        canBusOn(self._write_handle)
+
+        # timestamp must be set after bus is online, otherwise kvReadTimer may return erroneous values
+        self._timestamp_offset = self._update_timestamp_offset()
+
+    def _update_timestamp_offset(self) -> float:
         timer = ctypes.c_uint(0)
         try:
             if time.get_clock_info("time").resolution > 1e-5:
                 ts, perfcounter = time_perfcounter_correlation()
                 kvReadTimer(self._read_handle, ctypes.byref(timer))
                 current_perfcounter = time.perf_counter()
                 now = ts + (current_perfcounter - perfcounter)
-                self._timestamp_offset = now - (timer.value * TIMESTAMP_FACTOR)
+                return now - (timer.value * TIMESTAMP_FACTOR)
             else:
                 kvReadTimer(self._read_handle, ctypes.byref(timer))
-                self._timestamp_offset = time.time() - (timer.value * TIMESTAMP_FACTOR)
+                return time.time() - (timer.value * TIMESTAMP_FACTOR)
 
         except Exception as exc:
             # timer is usually close to 0
             log.info(str(exc))
-            self._timestamp_offset = time.time() - (timer.value * TIMESTAMP_FACTOR)
-
-        self._is_filtered = False
-        super().__init__(
-            channel=channel,
-            can_filters=can_filters,
-            **kwargs,
-        )
-
-        # activate channel after CAN filters were applied
-        log.debug("Go on bus")
-        if not self.single_handle:
-            canBusOn(self._read_handle)
-        canBusOn(self._write_handle)
+            return time.time() - (timer.value * TIMESTAMP_FACTOR)
 
     def _apply_filters(self, filters):
         if filters and len(filters) == 1: