Years and days are the only standard measurements of time that are “natural” rhythms of life (i.e. astronomical in origin). All others—months, weeks, hours, minutes, and seconds—are artifices, adopted for purposes of social coherence. So, it makes sense that they resonate more deeply and provide a more “organic” foundation for observance and commemoration. Still, We don’t normally count days (much less the other time periods) for an extended duration.
Just for illustration, 5000 days is about 13 ? years; 10,000 days is about 27 ? years; 20,000 days is about 54 3/4 years; and 30, 000 is just over 82. The oldest person alive currently (and the 4th longest recorded life ever), Lucille Randon of France, born on 11 February 1904, has seen 43,409 days.
According to the Bible, (Psalms 9:10) a full span of life is 70 years (~25,500 days). As I’ve noted earlier, health and demographics, particularly in the last 50,000 days (~ since 1870s), has lifted that target for most people on the planet. According to the Social Security Administration, I’ve got (on average) another 6,000 days or so
Days pass quickly enough and, almost as quickly, cycle into weeks and months. Every “older” person will tell you that time does seem to go by faster as one ages, but even in our youth these markers spin by, almost to no notice at all. Perhaps this phenomenon is related to the time dilation that Einstein posited in his theory of relativity; i./e., time slows down as we move faster.
Still, for newborns, counting by weeks seems OK for about half a year, then counting by months seems OK for 2-3 years. After we reach adulthood, for reasons of either boredom or fear of realization of age, we tend to shift to only paying attention to “big” birthdays, every five or ten years.
Whether days or years, round numbers tend to become occasions for celebration and reflection; both of which are good things, so I guess we shouldn’t pass up any convenient opportunity for either. I would invite you to bring over a cake, but I’m more of a pie guy.