Before the watch has been paired with your smartphone, it usually doesn't show the correct time and date. Once paired, the watch updates the date and time automatically, as soon as it connects to the mobile network.
If you keep seeing the wrong date or time on the watch, check if the SIM-card has active mobile data and if the watch receives good coverage. Rebooting the device will help the watch to reconnect to the network and update the date and time.
Keep in mind that it's not possible to change the time and date manually.