Dates keep changing

I use a memorized invoice to keep track of plowing events so that I can quickly bill my customers. I create an invoice and list each time I plowed and include the service date. Then I set the invoice for the last day of the month and then memorize the invoice. I will use this "template" for each of my customers for that billing cycle. It is now a 2 days into the new month. So I double click the memorized invoice from the mem. trx. window and it changes the invoice date to today which is fine, I can change that, BUT it also bumps each of the service dates ahead 2 days. Why would it do that and how can I make it stop???