Requirments:

1. most of feature of google calendar.
2. support calendars such as Chinese lunar calendar.
3. people can share their calendars as a calendar market.
4. people can share their calendars as a twitter way.
5. sophisticated notify system.

from: http://www.google.com/support/forum/p/Calendar/thread?tid=11a25616f198aec6&hl=en

1. we could create calendars without timezones, so that an event that if I create a calendar with an event that is all day on 12th November it is all day 12th November in any timezone and not their equivalent of what is the 12th November in my time zone. If this were the case people could create their own holidays calendars and share them.

2. When selecting the start or end time of an event "sunrise" and "sunset" are offered where possible. 

James