The best thing about using an invoice system is that you can track your time against the amount you're getting paid for each task. In order to run a successful business, you want your hourly rate to be profitable so you're not losing time and money. Another benefit of using an invoicing system is that you'll never save over an invoice and lose the work you've calculated for another client.
Due Date: The Invoice Date plus Payment Term Days give you the Due Date, which is in plain English and easier to understand. Having a Due Date on the invoice is a tactful and professional way of making it clear to your customer when they should pay.