by default, you can't
but there is a workaround
create a dummy file. there in the script check the actual time
if hour(now()) >= 8 and hour(now()) <= 18 then
// ok
else
error
endif
create a task every hour
then let your app run not on schedule but depending on successful run of the first task.
from 8-18 the task runs successful, your app is running, the other time ist runs to an error, your app is not running
