I use to develop a custom python calendar tools with web2py 2-3 years ago... We did pass a lot of the early calendar type issue like isoweek and sunday-to-saturday, I can't say it bugless from this side, but I am confident it always return proper year-month-week-day... I had event table (with attach CRUD form and even color management), with nice display of the event spread over multiple weeks or day and order them properly in order to the longer one be presented above all other to make sure it spread properly and you can keep track of "inline" continus event (hope what I explain it makes sens). It basically mimic office 365 calendar display back then. Mostly before launch a beta with the user I had to complete the js part which was controlling the calendar from the view... There were a couple of quirk that I could resolve (I am less familiar with js and I didn't code it so, it was more difficult to fix these issues). The main issue was that when you go from day to week to month it didn't keep track properly which day was selected. I don't recall if it was from day to month or month to day, in one way or the order it was working. So, there were not much work left to make a good fully working prototype... But I had to stop this project for different reasons... I am open to share it... Not sure if there would be interrest in it though. It lack proper intialisation parameters basically to remember where you left when you quit the calendar and get back to it. It all server side, but the js was loading months in advance back and forth in order to it to be fast, the page size couldn't get to big as we also taking care to delete months and didn't keep more then 3-4-5 month of data in the view. It is nice be requires a little love to shine...
:)
Let me know it is something worth share, if so I will try to extract the code from the main app were it nows back into a dedicated app and share it under CC BY-NC-SA licence if it comply with underlying licences...
RIchard