You can simply add a column in report definition and use function alias in that column for calculating the difference. You can use "pzGetAppHistoricalComplianceScore" RD as an example to see the use of function alias.
You can use the Difference in Minutes and multiple with 60 to convert the difference in seconds. For that First use the Multiply function to Multiply with 60, then use Difference in Minutes function get the difference. Kindly refer the attached screen shots for more info.
Adding to Praveen, to get the accurate difference in seconds: you can also try creating a custom function alias by doing save as of "Difference in Minutes" function alias and modifying the source in the function alias to get the result in seconds as below:
1. For SQL server: In DATEDIFF function modify interval parameter to 'ss' instead of 'mi'
2. For PostgreSQL: Remove '/60' for ROUND function to get the result in seconds
If you are using DB2 database, then this SQL source will pick. Please confirm if the same DB is used at your end. If not, in the function alias rule, from actions got to siblings action and open the function alias for your DB and make required changes. For DB2 DB, make the modifications as below:
Instead of TIMETSAMPDIFF(4,-----) try giving TIMESTAMPDIFF(2,----)
For Timestampdifff() function, first paramater describes the output unit.