1,000,000th triangle number. The sum of a million lives, ie 1+2+...+1,000,000. I'm another day older than my father, one day younger than my son is obvious if in reference to a number if we look at the set of reals. As a number it is larger than any which come before it, Towering over them, but to any larger number it is just one in the throng which come before. The set of reals is relentless and infinite. And when all is said and done, ie infinity, then all numbers which exist in R are in essence irrelevant, even a number as unimaginably large as that. The same as any, Its just a number, the story would be true for literally any number, with the only change perhaps being the sum of line. Meaning this is the only line which really hints at the answer, as all the others are true for numbers general.
A number in a summation? Each earlier number adds to it, and it will add to the next number. Each one is basically the same, and when they've all been summed none will be left. Day older and younger means number before and after.