Background Information
If you are using the Hours of work done per day or per week method for tracking progress, then each team member indirectly determines the Actual Start and Actual Finish dates of the task. The Actual Start date of a task is the first date on which any assigned resource reports time on the task, while the Actual Finish date of the task is the date on which the last assigned resource completes work on the task.
If you are using either the % Work Complete or Actual Work and Remaining Work method for tracking project progress, you will need to add the Actual Start and Actual Finish fields to your team members' timesheets in Project Web Access. Once you have done this, each team member can enter the Actual Start date and Actual Finish date of each task assignment, and this will impact the Start and Finish dates of the task.
Resolution
To add the Actual Start and Actual Finish fields to each user's timesheet in Project Web Access, refer to the following link:
Add or delete columns on the timesheet