The red number next to the clock shows time tracking from the past that has not yet been confirmed. 

By default, the time tracking from the last month is displayed.

Click on the clock on the left-hand side → then click on the circle (open time tracking) → now filter the period from 01/01/2000 to today. 

Then scroll down until all the time tracking is displayed.

You can now easily and quickly confirm the time tracking that is still open by pressing the button "Confirm all time tracking".