Time

Hi I need some help, so I have made an app that you can book rooms and I would like some with the Time picker please.

So basically, when a person chooses a start and end time that they would like to book that room, it would save it on the Listpicker, but lets say if another person chooses that same room, and they choose that same start and end time as that other person did, how could I make them not be able to choose the same time. 

Here are my blocks that I have started with

you will have to use a server database to store your data, for example a fusiontable to learn how to work with fusiontables, see the Pizza Party tutorial