I presently use an open source invoicing program called Myclientbase, It has a simple task module (to do list) this could be expanded (note the simple task mods has a lot of the features already built into it) or a new mod could be written. I need a calendar module ( a visible calendar using a number system for the weeks 1,2,3,4,5,6,7,etc) that allow me to book appointments (and or tasks) after I create an invoice.I must be able to transfer invoice information into the appointment (task) such as Name, invoice number and product. The information would display on the the dashboard and accessible through the side bar buttons. (Invoice and appointment or task is linked) Display or notify when appointment or task is due and allow a printout.
The flow would be, create an invoice, complete the invoice, be asked if you need to create a appointment or task, click yes, calendar appears the appointment or task is created, I have access to to the information through the dashboard and side bar buttons.