Set a valid date for the last ack variable.

Since the default value will always be well below the current time + waittime, waittime will always be ignored.

src/pygnssutils/ubxload.py

@@ -66,7 +66,7 @@ def __init__(self, file: object, stream: object, **kwargs):
  self._serial_lock = Lock()
  self._out_queue = Queue()
  self._stop_event = Event()
- self._last_ack = datetime.fromordinal(1)
+ self._last_ack = datetime.now()
 
  self._write_thread = Thread(
  target=self._write_data,