the process of time going by:
Most of the old traditions have died out with the passing of time.
The passing of the years had done nothing to improve his temper.
if you say something in passing, you mention it while you are mainly talking about something else:
He did mention his brother's wife, but only in passing.
the passing of something is the fact that it has ended:
The old regime was defeated, and few people mourned its passing.
the passing of a person is their death - use this when you want to avoid using the word 'death':
Nothing could fill the gap in her life left by his passing.