set fifteen_minutes_ago [expr [ns_time] - (15*60)]
set now [ns_time]
set sql_query "select count(distinct session_id) 
    as result from sec_session_properties where last_hit < :fifteen_minutes_ago"
set visitor_count [db_string select_sessions $sql_query]

ns_write $visitor_count
ns_write " $fifteen_minutes_ago $now"