E.g.: 1655472207557
Equals Friday, 17 June 2022 at 13:23:27.557
I used this website to convert it with: https://www.epochconverter.com/
The reason why I encountered this was that I was trying to figure out when we had a particular tradesman visit and I wasn’t having much luck with standard searches, so I pulled out the XML file from SMS Backup & Restore’s backups of my Android phone and it has those sorts of numbers listed as the date.