In certain circumstances you might be required to amend the purchase date on trade-in vehicle which is not directly possible to do from the trade-in vehicle record
Purchase date of a used vehicle record is held on purchase brief page (next to invoice number field). Please note that trade-in purchase invoice in Autoline is raised at the time of sales invoice for the main/prime vehicle and purchase date of trade-in is always copied from the sales invoice date of the prime/vehicle (held on Sales Brief page).
You cannot modify purchase date on a trade-in vehicle unless you have credited the sales for the main/prime vehicle. Therefore, for this to rectify, you'll have to raise sales credit for the prime vehicle, then amend the invoice date on the prime vehicle which will be copied to trade-in purchase invoice date when you print the invoice for the prime/main vehicle.