AI is really good at making things up. It has it’s uses, but it’s not always perfect. This is the same technology that can produce images of people with 16 fingers and limbs growing out of their head. Take it with a grain of salt. Both of the results you provided don’t make any sense within Glide.
Short answer is that it’s not possible to set a default on the fly within a native form. You can set it on the way in, but not while you are in the form. In most other cases, setting a value requires an action called by the user doing something.
Your best bet is to create a custom form instead of a native form. It’s more work, but gives you a little more flexibility. You still won’t be able to set a default end date based on a start date unless you have an additional button to write the start date to the end date…and you won’t have full control to restrict which dates a user can choose. What you will have with a custom form is the ability to create your own submit button with a custom action that first does some validation. If the end date is before the start date, then you can show a warning message instead of adding a row.