How this actually works
The first number is real math: your birthday, converted to seconds, counting up live. The second number is a guess dressed up as precision. It starts from an expected lifespan you set yourself, then nudges it up or down based on a handful of factors that genuinely do correlate with longevity in real research: dog ownership, regular exercise, smoking, chronic stress, and optimism all show up in actual studies. The effect sizes here are not those studies' effect sizes. They're rounded to whatever felt satisfying.
Once you start the clock, both numbers tick in real time, once a second, for as long as you keep the tab open. That part is not a gimmick, it's just what a countdown is supposed to do.
Should you actually trust the result?
No. Nobody knows how long they have, and a slider definitely doesn't either. This is a joke calculator on a website called Useless Calculators, not an actuarial table. Go outside.