According to the "Sterbebücher" (death records) of Auschwitz, Leendert Philip de Jong was murdered there on 17 September 1942. This date is different from the official date of death of 30 September 1942 as determined by the Dutch Ministry of Justice after the war. For the time being, the website joodsmonument.nl, only list the dates of death, as published at the time in the "Staatscourant" by the Ministry of Justice.
The date of death according to the death records (Sterbebücher) of Auschwitz
DThe official Dutch death certificate from the A-register 51-folio 30v, no. 172 dated 22 Sept 1950 (after clicking the link, please wait until the register openened itself after which you can view the death certificate at the left upper corner).