List files based on changes
Don't mix programming language
Try to avoid mixing different languages in one program. You are creating a Frankenstein monster this way, that depends on you having to deal with multiple languages, their interaction with each other, and now your computer needs to have the necessary interpreters and libraries installed for both languages. It is also quite expensive to call os.system(), so performance suffers as well.
For the rest of the review I'm assuming you want to want to continue using Python.
Make sure your script works with Python 3
Python version 2 is obsolete, you should move to Python 3. The most important change in your script though is adding parentheses to the print-statements.
Ensure you implement your program according to the requirements
The requirements you mention say that the directory and number of days should be passed as arguments. That means you should not ask for them while the program is running. The easiest way to parse command line arguments is to use the argparse module. As a bonus, this will also automatically create a help text that will be printed when you specify the --help option on the command line.
The requirements also say you have to sort the output based on modification time. Note that your original shell command did not guarantee that the output was correctly sorted: xargs might call ls multiple times, each time with only part of the input.
Have Python produce a list of files
You can use os.listdir() to get a list of all the elements in a directory. That list might also contain the names of subdirectories, but it will not recurse into them. Use os.stat() on each element to check if it is a file or a directory, and also to get the modification time.
Add a shebang line to the top of your script
Add a shebang line to the top, so you can call your script without having to call the Python interpreter explicitly. This line can look like:
#!/usr/bin/python3

I also recommend you remove the .py extension from your script, as it will be a stand-alone script, and for the user it shouldn't matter if it's writting in Python or any other language. This way, along with the argument parsing, you should be able to call your script like so:
$ ./main . 365
./README.txt                      2019-05-09  17:19:53.193771720  +0200
./README.txt.gpg                  2019-05-09  17:20:21.331833720  +0200
./migratemost-master.zip          2019-05-20  12:52:34.867119547  +0200
./INC177759                       2019-05-23  13:29:47.014557386  +0200

Of course, the name main doesn't really tell what your script does. Give it a better name, like list_recent_files.