This should start with middle school mathematics. In middle school mathematics, we use n to mean "1, 2, 3, ... until infinity". So the so-called "n days" and "n years" refer to a time range that can be described by natural numbers, but in the practical application process, people often emphasize the concept of "until infinity" and use "n days" and "n years" to refer to something that is far away and takes a long time.