BILLING SOFTWARE DEVELOPMENT PROJECT -- 2

I have some very basic billing software that was written for me several years ago by the spouse of my legal secretary, from, I think, open source. It is very basic and has many shortcomings. I need it to be able to keep and enter time for certain preferred clients who pre-purchase billable hours from me at a discount. Each month, their unused hours roll forward, accumulating to a maximum of three months. Unused hours expire after 90 days. I need an application that can be used on both desktop and on mobile, that lets me track attorney and paralegal billing hours, generated an invoice for the client, see at any point during the month how many accumulated hours a client has utilized and how many remain, keep track of participating attorney billed hours, and determine how much I owe to a participating attorney contributing to the work. I have a strong preference for a coder in the USA, Canada, or the UK. Will consider a merit bonus at the end of the project for work above and beyond normal excellence standards, completed expeditiously. Should be completed and operable, with deliverables completed, 30 days from acceptance. I have a detailed software plan and features list. An independent contractor agreement with non-disclosure will need to be signed prior to work.