Make a collection of Purchase Vouchers. Sort it on reverse Date ....so latest date voucher is on top. Then extract the first date from that.
[Collection: PurchaseLastDate] Type : Vouchers : VoucherType Child Of : $$VchTypePurchase Belongs To : Yes Fetch : Date Sort : @@Default : -$Date Please try this also as Amitji has said. I tried to make the Collection for you.