Jasper Duizendstra demonstrates how with a little bit of Google Apps Script you can easily use your Google Calendar to keep a record of how much time you spend on tasks and then query/report this via Google Sheets. The post contains instructions and code for getting started
Source: Record time and activities with Google Sheets, Calendar and Apps Script
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Member of Google Developers Experts Program for Google Workspace (Google Apps Script) and interested in supporting Google Workspace Devs.
Great in theory. Unfortunately though, it does not work.