adding item coverage production time in sales line date field

I have created a Date field on salesOrderLine , now when an item is added to a line auto populate the date field as

Date field = sys date() + Item coverage production time

I have added following code in salesline modified field along with my query as :

Case fieldnum(SalesLine, PlnDate):

this.PlnDate = today() + (How to call Item coverage production time from Item coverage form);

break;

This has been discussed here, http://dynamicsuser.net/forums/t/86292.aspx?PageIndex=1

you need to get this from \Data Dictionary\Tables\ReqItemTable\Fields\LeadTimeProduction

You are modifying the plnDate and updating it again after the modification?