Writting to file from if/else statement

[mcmxl22]

The original code is here.

The goal of this code is to have it change the two-letter code every day and write it to the file.

This is the problem code. Specifically lines 43 - 45.

import datetime
import os.path
import random
import string


DATE = datetime.datetime.now()
DAY = DATE.day

def transfer_file():
    """Check if file exists."""
    if os.path.exists("transfer.txt"):
        return
    else:
        with open("transfer.txt", "w") as file:
            return


def code_file():
    """Check if file exists."""
    if os.path.exists("code.txt"):
        return
    else:
        with open("code.txt", "w") as file:
            return

def set_expiration():
    """Sets date format and adds a day."""
    expires = f"{DATE.month}/{DAY + 1}/{DATE.year}"
    return expires

def set_code():
    """Sets random two letter code."""
    letters = string.ascii_uppercase
    code_1 = random.choice(letters)
    code_2 = random.choice(letters)
    day_code = f"{code_1}{code_2}"

    with open("code.txt", "r") as file:
        code = "".join(file.readlines())
        print(code)

    if str(DAY) not in set_expiration():
        with open("code.txt", "w") as file:
            file.write(day_code)
    else:
        return "".join(code)

transfer_file()
code_file()
set_code()

First I don't understand why it won't write to the file on the first run unless I change line 43 to if str(DAY) in set_expiration():.
If I leave the "not" out of line 43 it writes a different code on every run as expected. But if I add
the "not" to line 43 it will change the code on the first run but not on any runs after that no matter how I change str(DAY). I usually change it by taking the + 1 out of line 29. expires = f"{DATE.month}/{DAY + 1}/{DATE.year}"
Why does it ignore my changes?