ETC cannot exist previous to the Actual Thru date of an assignment. In this example, the assignment has not posted actual to this particular task, but has posted actuals to other tasks on the project. When this was done, the Actual Thru date on all of his existing tasks updated to the last date of that time reporting period (in this example: 5/23) and all of the ETCs on the tasks were pushed forward to start on the first workday following this date. Since this task had no actuals to hold the Start Date, the Start Date moved forward with the ETCs as well. While this Actual Thru date field is populated, the task and assignment Start Date will not be able to be moved to an earlier date.
In order to temporarily clear out the Actual Thru date on the assignment, the assignment would need to be removed from the task, and readded. Once the assignment is readded, they will no longer have an Actual Thru date and the Task Start Date can be updated to any earlier date.
However, in order for this earlier date to remain the Task Start Date, the resource would need to post actuals for this date. If they do not post time for this particular task, but do post time for other tasks in the system, their Actual Thru date will update, and the Task Start Date will be pushed forward again.