Your question does not appear to be related to the thread you have posted, so you would have been better to start a new thread. However one possibility is shown below. If you have further questions please start a new thread.
In a function node you can use substring to extract the first 10 characters of the datetime string. Assuming the data you show is in msg.payload then you do
msg.payload.Ablauf = msg.payload.Ablauf.substring(0,10)
Example
[{"id":"9105fd5fb3a348c3","type":"inject","z":"bdd7be38.d3b55","name":"","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"{\"Nr\":4,\"Inhalt\":\"Fish\",\"Menge\":\"500\",\"Fach\":7,\"Ablauf\":\"2023-12-30T23:00:00.000Z\"}","payloadType":"json","x":360,"y":2000,"wires":[["95693fda06b53eff"]]},{"id":"95693fda06b53eff","type":"function","z":"bdd7be38.d3b55","name":"function 3","func":"msg.payload.Ablauf = msg.payload.Ablauf.substring(0,10)\nreturn msg;","outputs":1,"timeout":0,"noerr":0,"initialize":"","finalize":"","libs":[],"x":510,"y":2000,"wires":[["f226f8db207e1cc4"]]},{"id":"f226f8db207e1cc4","type":"debug","z":"bdd7be38.d3b55","name":"debug 100","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":710,"y":2000,"wires":[]}]