Adding function to online booking script.

We run a web design firm for hair salons. We have added an online booking calendar, however we have had requests to add “service” to the script. This allows their clients to book specific services that the salon offers. We have added services, however each service (stored via mysql) has a duration, which is also stored via mysql. We need to apply the duration to booking time. Example if someone chooses a service that is 45 minutes long and wants to start at 9:00 it will book from 9:00am to 9:45am then allow someone else to book from 9:45 to whenever depending on their service selection and it’s booking time. So we just need to somehow allow the duration to be applied to booking start time and not allow any overlapping of bookings due to service length.