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davx5-ose
DAVx⁵ is an open-source CalDAV/CardDAV suite and sync app for Android. You can also access your online files (WebDAV) with it.
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
I also wanted to avoid Google calendar and ended up with this setup: I have a little linux server home with Nextcloud running on it, where caldav and carddav can be used. In order to sync the outlook calendar at work with the home server, I installed a sync tool to work PC: https://caldavsynchronizer.org/.
With this setup, I can sync my calendars and contacts between personal devices and work pc. With iphone, one can use cal and carddav out of the box. I remember using https://www.davx5.com/ on my android phone last year. I am not sure if it would still be necessary though.