🔧 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...