Dependency injection is another way to do it - just passing in something that gives you the time (e.g. a function).

Here are some example tests:

import datetime

from greeter import greet

def test_it_returns_good_morning_before_12():
    def get_time():
        return datetime.datetime(2024, 12, 8, hour=5, minute=0, second=0)

    greeting = greet(get_time)

    assert greeting == "Good morning!"

def test_it_returns_good_afternoon_after_12():
    def get_time():
        return datetime.datetime(2024, 12, 8, hour=14, minute=0, second=0)

    greeting = greet(get_time)

    assert greeting == "Good afternoon!"

The greet function then calls the function it is passed to obtain the time:

def greet(get_time):
    hour = get_time().hour

    if hour < 12:
        return "Good morning!"
    else:
        return "Good afternoon!"

In the production code where I need to use greet, I'd simply pass the function that provides the system time:

greeting = greet(datetime.datetime.now)

I did write a tutorial about this over on python-forum: https://python-forum.io/thread-38222.html. That does include a link to a repo on GitHub with a more complete example (https://github.com/ndc85430/controlling-date-and-time-in-tests).