ProPresenter's Clocks has a feature called "Overrun". This option allows your clocks to run past 00:00 and show a negative time when someone has run over their time limit. If you are already using Clocks on your Stage Display, you only need to configure a couple of settings for this to work.
Open the Clocks module and enable Overrun for the clock you want to use this feature with. Keep in mind that if you are using the same clock within Messages for some reason it will also show the overrun time.
Once you have enabled the Overrun option, open your Stage Display layout settings. You don't have to make any changes here, but if you want to set the Overrun color to a different color, you can.
Once you have it working, you can test your clock. It should look something like this.