Start time isn't problem - just select INSERT - DATE AND TIME ... from the menu and it'll do it. You could do it by creating a placeholder column and set its value using the "before_report" trigger as
:placeholder_column := sysdate;
Notice that this shows time in the database, not on your computer. Of course, format it at the output to show time (hh24:mi:ss, for example).
But, showing the time when your report ended isn't that easy. There is the "after_report" trigger which could be used to perform this action, but this trigger won't fire when you'd like it to ... read more about it in Help section of your Builder. I tried but didn't succeed to show end time. Therefore, I'd be glad if someone managed to do it and could share this with us.