I propose the zero(time_unit) function, which replaces all time units with greater equal resolution than time_unit by 0.
E.g. 
>>> datetime.datetime(2020, 4, 18, 12, 27, 30, 500).zero("second")
datetime.datetime(2020, 4, 18, 12, 27)
I purpose it for the datetime, time classes.
I also added unit tests for the function in those classes.
GitHub PR:

https://github.com/python/cpython/pull/19580

What is the use-case for this new method?

The use-case for this method is to "limit" the resolution of a time object.
I encountered the need for it when I needed to reduce the resolution of some timestamps, to compare them with timestamps of lesser resolution.
I found the following SOF post: https://stackoverflow.com/questions/13838394/can-i-easily-get-datetime-with-less-resolution-in-python
which made me think that a zero function can be nice for that use case of datetime.replace

I recommend posting this to python-ideas for discussion.

FWIW, I like the answer in the StackOverflow link better than this proposal.

At any rate, I think Terry's suggestion is a good one.  If you would like to move forward with this, please take it to python-ideas.

That said, if another core-dev wants to champion this, feel free to re-open.