You can set the starting time of your SessionLab agenda on top of the Session planner, right above the title, however you might also want to design your agenda within a specific time frame, perhaps because you need to end your session at a particular time.

To show this in SessionLab, you can create a time-locked block with no duration at the end of your session. You'll be able to plan your session keeping the 'unused time between blocks' notification as a guide to how long remains in your allotted time.

This video will give some more information and examples on how to achieve this: