Because so much is already automatically tracked in my calendar (e.g. the HR system adds calendar events for booked holiday), I just needed a way to query it to get the metrics I wanted.
Originally I used a Go MySQL implementation to evaluate queries but it was a bit buggy and didn’t support all the SQL I wanted. Now I’m using a full SQLite database for each user so you can really do some gnarly queries (recursive CTEs, window functions, the works).
Personally, I’m using QueryCal as a Grafana datasource to power a dashboard that’s displayed on an old Kindle on my desk (using this great project: https://github.com/pascalw/kindle-dash).