Comparing the baseline and actual schedules
Once you have finished setting up a project and you're ready to start implementing it, you can set a baseline schedule. The baseline represents the original intentions of the project, for comparison with the actual schedule. For example, consider this simple project:
Choose Split Schedule or Both Schedules to see the baseline and actual schedules together, so you can compare them. 
Once you click the Set Baseline button, the baseline schedule is decided and any further changes are applied to the actual schedule instead. 
The "Set Up Equipment" task took less time than expected. When you change the Actual task, the Baseline task remains visible below it for comparison. The Showtime milestone has updated its time. You can check the original time by switching to the Baseline schedule, or looking in the Task Information inspector. To revise the baseline schedule after it's already been set:
Select the tasks to revise.
Click the Set Baseline button.
Select the Set only on selected tasks checkbox and click OK.
The selected tasks' Baseline schedule updates to match its Actual schedule.
You can also modify individual tasks' baseline dates in the Task Information inspector.