I'd like to offer users of my web platform calendars, and also offer group calendar and public calendar features. I know of Google Calendar API, of course, but that would require all my users to have Google accounts (although, I guess the question is who does not these days). I am wondering if there are any other alternatives? I can write my own, but this has gotten to have been done thousands of times already!

I am looking not so much at the front end; we'd probably going to re-skin the UI anyways. But I want a solution that has good backend support for storing events and recurring events, and maintaining multiple calendars. Ideally, the engine would also support things like shared event between different calendars, syncing with other calendars, etc.

Sad thing is that this does seem like Google Calendar would cover it, but asking all the users to have to have google accounts may not be realistic. Especially since we'd also want to create group calendars, and I am not sure under who's google account that'd be, and if that'd even be covered under their terms of use.

Your thoughts?

Thanks.