Off-Topic, highly opinionated comment: That’s a GeneXus control, I believe. A system designed to provide code as well written as a developer (so they claim). No good developer would mess around with onblur inside an onchange handler (or the reverse) and then call yet another onchange handler. That’s just terrible and tells me everything I need to know about this library/company.
Your problem is likely caused by that mess I mentioned above. To help you, I went to the GeneXus site to dig out the correct API to apply a value from code. In a reasonable amount of time (a little more than I intended) I failed to find the API docs. All I got was marketing noise and other bullshit. If you know where they are, post a link, please.
So, we’ll need to try some experiments. This might not be quick. Maybe we will find a solution on the first attempt, maybe not.
So, experiment 1:
What happens when you paste this into the DevTools console?
document.querySelector("input[name=SDTVAR_SDTPERSONA_KLIFECNAC]").value="24/10/1985"
In your response, post a screenshot of the browser with the page loaded and make sure the DevTools are visible.