Do you have course completion tracking turned on in the course(s)? Is it set? If so, there is a table mdl_course_completions with a timecompleted field. This field is, as are most time/date fields in Moodle, a unix timestamp and may need conversion to a human readable date, depending on how your BI works.
Or...you could just download the course completion report which reports completion dates for each course which lists all enrolled users.