All manual changes to a task’s schedule are recorded in the project and task history (in the Activity tab).  To trace schedule changes propagated from a parent task, check the Activity feed of parent tasks up the task hierarchy as well.

You can also visualize the changes in the SCHEDULE view.

In the Schedule view, select the “Track Changes” button on the Task toolbar. The previous schedule will be gray with a dashed border on the Gantt bar:

To clear the history of changes to a task, hover over the Gantt bar to reveal the menu and click the “Clear History” button: