Imagine suddenly you no longer exist, poof, from one day to the next there is no more you.
What if you knew that this day would come in a month?
What if it wasn’t for another year?
Or in 5 years?
Would you spend your remaining time differently than you do now?
If so, why aren’t you already doing it?
There is no way to know whether changing the way you live would still kill on that death date. You wouldn’t know whether because of the changes you make, you die on that day. Also you wouldn’t know if not making a change kills you earlier.