It's estimated that the U.S. population is about 300,000,000, and the point estimate for when this milestone will be reached is Tuesday.
I guess because I was poorly socialized as a kid, I've never understood why some things are milestones, and others are not.
Watching your car odometer hit 44,444.4 seems, to me, just as exciting as turning 40, but somehow the odometer change excites only boredom and turning 40 leads mid-life crises, or at least gag gifts.
I once bought pizza for my department when I had spent 100 years in dog years at a company (14 and 2/7 years * 7). This was regarded as eccentric, but was at least accurate. The 300,000,000 milestone is just a statistical estimate, and there's no realistic way to know when this milestone wll actually take place within a couple of years, given the limitations of population measurement.
By the way, I don't suggest using this line of argument for events such as your wife's birthday, or your wedding anniversary.