Actual start date and actual end date in cprojects

Hi all

Can anyone tell me what is the significance of 'actual start date ' and 'actual end date' for a project in cProjects in the table CGPL_Project.

As per my interpretation actual start date is the date when project status is set to released and actual complete date is when the project status is set to completed.

however these fields are not getting populated in the table , inspite of completing a project and setting it for archival

pls. advice

regards

Kedar