Sounds like you have a date field with an auto-enter calculation expression that doesn't change values when you then edit the Estimate field in the transactions table. This is a known limitation of auto-entered calculations. Changes to fields in related tables will not cause the field to update.
Define a calculation field in LineItem that returns a date type and this should work for you. The resulting value is unstored, and this can result in delays if you need to refer to this field in sort or find operations, but it should work.
If you need this value to be stored, you'll have to set up a script trigger on the estimate field that updates this date field in LineItems.
I didn't set it up to auto enter with the calc. It just won't eval the field. Your soln will work so I'll have to write a script to enter the data into a stored field.
I also found a work around, I created a new calculated field in the Transactions table and inserted it in place of the lineitem date field in the table (portal) . This evaluates the estimate field correctly and places or dosen't place the date as needed. This will allow me to print an estimate of charges.