I am building a shuttle scheduler for our hotel and I want to enforce functionality with in it as follows:

1) I want the program to look through the database when a user inserts a shuttle run and look to see what other runs are for the day, and based on that information and the time of day that a guest is asking for it will either let it go through or kick back a message saying that their is not enough time. For instance, a shuttle run at 8:30am to a company that takes 30 minutes round trip (roundtriptextbox) and a guest wants a shuttle at 8:45am, it would say "Shuttle Time Not Allowed, Shuttle will be out!" or something like that because often times our front desk agents will schedule shuttles at any time with out qualm that the shuttle won't be back in time to pick them up. Any suggestions? Have I made it clear enough? I am using VS2010