The time bar located at the top of the software helps with visualizing the time at which events occur in your capture. In addition, right-clicking on the time bar will allow configuration with regards to how time is displayed.
The time point at which t=0s occurs can be configured to the following events
Start of Capture: Sets the start of the capture as t=0s
Digital Trigger: In a digital triggered capture, this sets the point at which the digital trigger occurs as t=0s
Analyzer Trigger: If the trigger was the result of an Analyzer Trigger (located under the Analyzers panel > Trigger View), then the point at which the trigger occurred will be t=0s.
Timing Marker: If any timing markers are present, then the selected timing marker will be set as t=0s. As an alternative, you can also right-click on your timing marker and select "Set as t0" in the pop up menu.
Wall Clock Time: Changes the time format to your OS wall clock
The Time Preferences may also be opened for further customization on the time bar behavior.