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.
Using invoicing systems to make it simple - While you can make up your own invoice or use a free template, it's often easier to use an invoicing system that offers more in depth features. Some invoice systems, like Harvest, offer the option to create estimates for big projects, keep track of multiple clients and send reminder emails when payments are late. Another option is Freshbooks, which offers similar features but includes profit and loss sheets, as well as trackers to help you manage your time.