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Online Calendar Software
(sopuli.xyz)
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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I second that, been using mailbox for the past 6 months and it works like a charm. You also have a todo tool on mailbox that I think synchronizes via Caldav too, but I don't use it so I don't know how it compares to todo.txt
It does, yes. I use the tasks.org app to manage.