The SL1 has its own internal Real-Time Clock. Using an NTP (Network Time Protocol) server, the SL1 is able to update the printer's set time when connected to the internet. This time is used to provide you with an actual estimated time of completion, not just the running time of your print.
If the printer is connected to the internet, you can make sure that you always have the correct time by entering:
Set 'Automatic time setting' to ON (green square). Make sure that the timezone is set correctly. If the printer's time or the timezone is incorrect and the 'Automatic time setting' is OFF, the estimated end of prints will be based on the incorrect time.