DATEDIFF on SECONDS results in OVERFLOW, after a few months of data

DAX DAX Calculations
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I would have suggested that you calculate the duration in Power Query. However, I notice your data connection is DirectQuery from SQL Server? Have you considered calculating duration (in secs) and duration in hours in SQL Server?

The issue might be with DATETIME datatype. You might want to consider using Transaction Duration (sec) column in your visual. A Duration Datatype would have been ideal.