Ask your smartphone how to drive from Copenhagen to Berlin and it will give you an estimate of how long the trip will take, based on current traffic. If there is a traffic jam in Hamburg, say, the extra time this traffic jam takes will be included in the estimate. But, of course, you are not at all the points of your journey now. Rather, you’ll be in Copenhagen first, then at Odense, then Kolding, and so forth. By the time you get to Hamburg, there may no longer be a traffic jam. The estimate your smartphone gave you will be off. Life expectancy is calculated in much the same way.
Life expectancy in 2019 is calculated using the chances of survival for all ages in 2019: those who turned 70 in 2019, those who turned 69 in 2019, those who turned 71 … you get the point. But nobody actually has all their birthdays in 2019. People have at most one birthday a year (less than one for some of those who died that year and those born on February 29). Since I turned 35 in 2019, why should the 2019 chances of survival for a 70-year-old matter to me? By the time I turn 70, the world will have changed. The estimate will be off.
But your smartphone also tells you something like “31 minutes extra travel time due to a traffic jam”. With this information, you can guess how long the trip will take assuming that the traffic jam will be resolved by the time you get there: just subtract those 31 minutes. Every part of the journey has a travelling time and you can pick those pieces apart.