All times are stored as UTC. They're converted to local time based on the time zone of the local machine.
In PowerShell, the creation of a DateTime object, by default, converts the time to local time unless you explicitly ask for UTC. If you write a DateTime object to a file, the time is NOT converted to UTC. That's something you must do unless the DateTime object's already in UTC.
You can use the "Kind" property of the DateTime object to discover if its UTC or Local.