Startup Idea : Share-planner

I would like to develop an app that works like a diary. The idea is simple. For instance: You're making a dentist appointment. The secretary will put the appointment in the diary. At the same time the appointment will appear in your diary too. This could help alot of people that forget important appointments or that are just to lazy for putting it in their diary. This app could be used in alot of different environments. Like in business or at other simple thing like a hairdresser for instance. I'd love to hear some feedback on this idea. Thanks