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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.
Check out Event Calendar widget. It has a ton of options over the default clock widget and I believe it's possible to load up a calendar without relying on akonadi or kdepim and get notifications for it that way.
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.