you already know about the end date will be 7 day after startdate. you can call a store procedure when you need to insert the data to database, in the store procedure you can make it automatic add 7 day on the end date. this is another way to insert but the result will be same.

i also have another idea, that is using javascript or ajax to add it automatic. after u key in the startdate, you add in a event on start date and when you key in the start date. it will call up js and javascript to assign a new value to end date.

hope it can help you