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 Converter — Epoch to Date, UTC & Local, Free

Free Unix timestamp converter. Live epoch clock, auto-detect seconds vs milliseconds with the assumption shown, UTC and local time side by side, copyable ISO 8601, and date to timestamp in seconds and milliseconds. Fully local — nothing uploaded.