Tag history time stamps are in milliseconds from epoch.
In theory, if you weren't concerned with potentially loosing data, then you would probably use, system.date.format() to provide a formatted date string, but it would really depend upon how you are generating your CSV.
Those are Ignition's timestamp values which are in milliseconds since 1970-01-01 00:00:00.0 (the UNIX epoch). Ignition has several functions to work with date and time. See system.date.format | Ignition User Manual, for example.
Ifeanyi, please do not cross-post. It wastes people's time, causes duplication of effort and is against the forum rules. See FAQ - Inductive Automation Forum.