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That kind of depends on what kind of report you would want it to produce. If you wanted to count the number of times the slip occurred, you could create an integer field in Project, then populate it with a Business Rule that increments the number when Planned End Date changes to be later than the previous Planned End Date, AND state = WIP or active = true, something along those lines.
There is OOB functionality available for calculating project slippage.
Project manager can configure status reports for project to show project slippage.
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