CALENDAR AND DATE PICKER

we have very serious problems with a coder. this guy allegedly fixed the latest jquery date picker calendar for us and helped us connect it with our booking engine, but we are getting customer complaints every day and he is costing us lost business. WE NEED THIS FIXING. we will pay 600 euros for this simple job, but will not pay out until 60 days have elapsed following installation, during which the date picker and calendar must be proven to WORK FLAWLESSLY WITH NO ISSUES ON ALL DEVICES IN ALL BROWSERS. we'll escrow the money, but we need evidence of TOTAL FLAWLESS FUNCTIONALITY.