Duration of WBS elements

Hi friends,

I am maintained activities & WBS elements in a Project. In this I am doing strict Bottom up planning & Forward scheduling.

The dates reflected in the WBS elements are OK. The Duration of the WBS element is taking Earliest Start date & Latest Finish Date.

My client wants that the Duration of the WBS should consider the Earliest Start date & Earliest Finish date.

Is it possible? or I have change the standard program.

Kindly suggest.

Regards,

Abhra.