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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.
Backend for the tasks is CalDAV, in my case Nextcloud. Front end on my phone is Tasks (from tasks.org)
I like to use http://todotxt.org With some file sync Tools. It has quite a few open source apps that support it. (For syncing i use syncthing and (sadly) Dropbox)
NOTE:
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Hence, a higher number means a more popular project.