Required delivery date not to be cpoied into sales order from quotation

My requirement is that required delivery date should not be copied when i create a sales order with reference to quotaion,

In the sales document types i have unchecked propose delivery date .it works fine while creating quotaion but then when i create sales order with ref to quotation the required delivery date is copied into the sales order which i dint want.

Kindly guide me on this