Carbon add method

Certainly something trivial, but I’m stuck!

I have a start date initialized like this:

$startDate = Carbon::createFromFormat('Y-m-d H:i', '2024-02-01 08:00');

I add days to this date to get an end date:

$qtyDays = 25;
$endDate = $startDate->addDays($qtyDays);

What I don’t understand is that the end date is calculated correctly. But the start date contains the end date.

How not to touch the start date ? Even though I read the Carbon doc, I don’t understand what’s going on.