Email Per Hour Report to calculate the productivity of the Email Agents

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Hello All, i am new to the world on Oracle CRM. I need help understanding the formula for one of the columns for "Seconds Logged in" in a report that preexisted in our system to calculate the productivity of email agents. This report is used to calculate the productivity of an  Email agent . i am also attaching the definition of the report. 

This is the formula: sum(date_diff(if(date_diff(user_trans.end_dttm, $end_time) > 0, $end_time, user_trans.end_dttm),if(date_diff(user_trans.start_dttm,$start_time) < 0, $start_time, user_trans.start_dttm)))