My theory is that about a year ago, there was a lot of buzz (possibly true, possibly false) about Google shipping a calendar, and everybody thought, oh gosh, it's gonna be really good, like Gmail, and then Yahoo! is going to be embarrassed again, and run out and buy the best Ajax calendar company they can find, just like they did with Oddpost, making those very funny kids millionaires overnight. --Joel
This may be true for some, however, I think there may be a less financially motivated reason as well. Everybody thinks of time differently. Just check out the comments on a recent entry on Signal vs. Noise. All 80 commeters had their own ideas about how time should be represented. It looks like many believe in the time-tested grid calendar design, but greatly differ in how it should be presented. Some like very detailed event listings showing your schedule in minute intervals. Others prefer simpler layouts with time represented by position. Some use it for business, while others for planning their weekends. Some people like to see exactly 3 weeks out, and some like an entire month. Others want the current day to always be in the middle of a 30 day grid, while others want more of a conventional calendar month.
Isn't there a possibility that the authors of these Ajax calendars are doing it because they aren't satisfied with what is currently available? What if their solutions works fine for them and they wanted to share it with everyone else?
Simply put, the calendering problem is very hard. It is impossible to please everybody. Joel listed what he wants in a calendar. Thats nice. But ask someone else and they'll give you a completely different list. I believe we need a few solutions depending on need. This is fine as long as they all play nicely using SSE, RSS, and/or iCal.