I know this may not be exactly what you are looking for, but I thought you might at least like to take a look at it if you didn't already know about it. Google has an online calendar program that works is pretty much any browser: http://www.google.com/calendar
. Its no open source, but.
You might also want to take a look at the Mozilla Sunbird
project. I've used it both on Windows and Linux. You can choose where the calendar file is saved and as long as both Windows and Linux can read/write the partition, it might be just what you're looking for.