How unix epoch milestones works
Unix time is one long count of seconds since 1970-01-01 00:00:00 UTC. Round numbers of that counter land on specific, computable dates: the billionth second fell on 2001-09-09, the two-billionth lands on 2033-05-18 at 03:33:20 UTC, and 2,147,483,647 — the largest count a signed 32-bit clock can hold — arrives on 2038-01-19 at 03:14:07 UTC, the edge of the famous year-2038 problem. The tool lists the past milestones and counts down to the future ones from any instant you give it.
Add a birth date and the same arithmetic turns personal. You get your exact age in seconds and days, the precise UTC instant of your billionth second — a gigasecond, which falls around age 31 years and 9 months — your 1.5- and 2-billion-second days, and the round-day anniversaries from 10,000 days (about 27 years 4 months) up to 40,000.
Everything is plain arithmetic on UTC instants, so there are no time-zone surprises in the milestones themselves: 2 billion seconds after the epoch is that instant everywhere on Earth. The one convention to know is that a birth date without a time is taken as midnight UTC, which shifts second-level anniversaries by up to a day if your real birth time differs — add the time if you want the exact instant.