Whether you have a local time that needs a time zone attached, or a time from some reference point that might need converting into a local time for display, depends on where you got the time from. I would generally expect a file modification time to be a reference time that would need converting into local time for display purposes, but only if you actually need to display it. If you're (for instance) comparing it to other file modification times, or using it as a git commit timestamp, you don't need to do any kind of conversion.
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