Both directions, both timezones
A Unix timestamp is a single number standing for an exact instant, which is perfect for machines and unreadable for people. This tool converts either way — timestamp to a readable date, or a date back to a timestamp — and always shows the date in both UTC and your local timezone so there is no offset arithmetic to do by hand.
Seconds or milliseconds, sorted out for you
One of the commonest bugs with timestamps is mixing up seconds and milliseconds — a factor of a thousand that lands you in 1970 or the year 50,000. The converter detects which unit you pasted and handles it, and when producing a timestamp it gives you both so you can copy the one your system expects. It all runs in your browser.