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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.
If you want FOSS, and just a simple calendar app thats standalone, you might like Borg, which looks like its getting regular updates and still supported https://github.com/mikeberger/borg_calendar
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.