Report Timing Configuration is used to specify how data should be calculated across parent and children issues. This configuration is used to rollup dates and for calculating percent complete.
For example, let’s say an epic’s dates are set one month apart, but its stories span 2 months. How should we represent that epic’s timing?
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The Report Timing Configuration lets you specify how values should be handled between each level of the issue hierarchy. For example, if you want to report only the epic dates, you can pick that; or if you want to only report the story timing, you can pick that. There are other options as well.