Looks pretty cool, but I would've expected a calendar for the 21st century to either be a mobile app or at least be mobile friendly on the web. Instead, I was greeted by a message telling me to visit from a desktop computer to download.
Although it might looks like a weird design choice, an installed app was the only possible choice not to store the user credentials on our servers. I have elaborated this in another post.
You have localStorage and indexedDB on the web. Web is a pretty powerful client.