In the job.end set it to type formula with
field('job.start') + seconds(random(1,59)) + days(random(1,30)) + month(random(1,12)) + year(random(1,8))
In the job.end set it to type formula with
field('job.start') + seconds(random(1,59)) + days(random(1,30)) + month(random(1,12)) + year(random(1,8))