Why `new Date(garbage) === "Invalid Date"` is always false (a timestamp converter taught me this the hard way)

I built a small Unix timestamp converter — paste in seconds or milliseconds, get a date back, or go the other way. Simple enough that I figured the only real work would be the date math. Then I went to add "tell the user when they typed garbage" and ran straight into a JavaScript gotcha that's been quietly living in more codebases than anyone wants to admit.

The unit dropdown is secretly a multiplier, not a branch

The converter has a unit selector next to the input — "Seconds" or "Milliseconds." The tempting way to wire that up is an if/else: if seconds, do this math; if milliseconds, do that math. That's not what's in the actual component. The <a-select> options are bound directly to the numbers 1 and 1000, and those numbers get used as literal arithmetic operands everywhere downstream:

const unixToNoraml_getOutput = () => {

let unix = data.unixToNoraml.unix;