How does it know seconds from milliseconds?

In Auto mode, values of 1,000,000,000,000 or more are treated as milliseconds and smaller values as seconds. The assumed unit is always displayed and can be overridden.

Why do UTC and local time differ?

A Unix timestamp is one instant in UTC. The local line renders the same instant in your device timezone, so the clock reading differs by your UTC offset. Both lines describe the same moment.

Is anything uploaded?

No. All conversions run in your browser tab. Nothing is uploaded, stored or shared.

Unix-Timestamp-Konverter — Epoch zu Datum, UTC & lokal, kostenlos

Kostenloser Unix-Timestamp-Konverter. Live-Epoch-Uhr, Sekunden/Millisekunden-Erkennung mit sichtbarer Annahme, UTC und Ortszeit nebeneinander, kopierbares ISO 8601 und Datum zu Timestamp. Voll lokal, kein Upload.