doc/paper/listings/add_visit_duration.rb
# /lib/abrupt/transformation/client/visit.rb
def add_visit_duration
leavetime = Abrupt.parse_time(@values[:leavetime])
enterteime = Abrupt.parse_time(@values[:entertime])
return unless leavetime || enterteime # maybe no recognized leavetime
visit_duration = (leavetime - enterteime).to_f # in seconds
add_data_property('visitDuration', visit_duration)
end