fix: Create timezone-aware datetime for Objects

[frereit]

Use .fromtimestamp instead of .utcfromtimestamp to create a timezone-aware datetime object.

Previously, the object was naive. .utcfromtimestamp is deprecated because naive datetime objects lead to ambiguity and problems.

By using a datetime-aware object, is is clearly signaled to the caller that the datetime is a UTC-timestamp, and it may easily be converted using .astimezone

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