I recently came across this error "Difference of two datetime values caused an overflow at runtime" when I open the performance_dashboard_main_report.
Thanks to Keith Elmore for providing the fix and allowing me to post this here.
This is a caused by one of the queries in the report that attempts to do a datediff for a session that has been connected to the server for longer than ~20 days. This can be fixed by editing the MS_PerfDashboard.usp_Main_GetSessionInfo procedure in setup.sql script as follows:
sum(convert(bigint, datediff(ms, login_time, getdate()))) - sum(convert(bigint, s.total_elapsed_time)) as idle_connection_time,