Janssen has been given a death date as late as 1638 although his sister said he was dead in 1632 testimony and his son Johannes declared his parents had died by the time of his marriage in April 1632.
قيل أن يانسن توفي سنة 1638 م، رغم أن شهادة أخته أنه توفي سنة 1632 م، وتصريح ابنه يوناس أن أبويه كانا قد توفيا قبل زواجه في أبريل 1632 م.