utils: stop using datetime.utcfromtimestamp() deprecated in Python 3.12
Python3.12 made tests fail with warnings:
DeprecationWarning: datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.fromtimestamp(timestamp, datetime.UTC).
Computing the diff while in timestamp seconds seems to preserve to the original
intent from ae04af1ce78d.
It would be nice to have some doctest coverage of this, with the problematic
corner cases that has popped up over time...