If you're incorporating date and time data into a website, it may not make sense to roll your own calendar system. Using someone else's service can cut down on development time and may give you more features than you'd have if you did it on your own.
Here are three free APIs to look into if you want to build a calendar or event tool.
30 Boxes - The basics are covered: programmatically add, retrieve, update and delete events on this popular calendaring site. The API also lets you search events by full text or tag. 30 Boxes also has todo data available through the API.
Google Calendar - Does everything that 30 Boxes does, minus the todo stuff. It's hard to deny the power of ubiquity. If your app needs to access your user's data elsewhere, you'll probably use Google Calendar.
Spongecell - This event promotion service also has a surprisingly complete API. If you're looking to create private calendars, this might not be the one to choose. If you organize events yourself, check out Spongecell. In addition to normal calendar stuff, you can send invitations, check on responses, and more.
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