What to run
Open Unix Timestamp Converter. It stays in the browser.
How to read the result
Seconds and milliseconds are both shown, plus UTC and local.
10 digits is seconds. 13 digits is milliseconds.
If the year looks like 50000, you treated milliseconds as seconds.
What to open next
Open Epoch Now to compare with now. It stays in the browser.
Open Time Zone Converter if the stakeholder wants a named zone. It stays in the browser.