append-at.py

#!/usr/bin/env python3

# argv[1] is the path to a configuration file
# the config file is a CSV file of the form date,path,message. EG:
# 03-03-2022,/home/user/py/remind/f,remember to do a backup
# 10-22-2021,/home/user/.motd,get tickets for Dune

from datetime import datetime
import os
import re
import sys

def main():
    data = read_file(sys.argv[1])
    # write back lines that we don't take action on to conf_output
    conf_output = open(sys.argv[1], "w")
    valid_line = re.compile('\d\d-\d\d-\d{4},.+,.+')
    data = filter(valid_line.match, data)
    for row in data:
        fields = row.split(",", 2)
        # >>> "05-15-2021,/home/user/motd,steal the vase, Indy!".split(',', 2)
        # ['05-15-2021', '/home/user/motd', 'steal the vase, Indy!']
        # note: a comma in the path will cause a bug
        date_string, file, message = fields[0], fields[1], fields[2]
        dt = datetime.strptime(date_string, "%m-%d-%Y")
        if datetime.now().date() >= dt.date():
            try:
                append(message, file)
            except Exception as e:
                print("[!] caught an error while appending:", e)
                print("[!] line that errored:", row)
                conf_output.write(row)
        else:
            conf_output.write(row)

    # if we crash before this point we lose the config data
    conf_output.close()


def append(message, file):
    file = file.replace("~", os.getenv("HOME"))
    # we should test whether file is already open
    f = open(file, "a")
    if not message.endswith("\n"):
        message = message + "\n"
    f.write(message)
    f.close()

def read_file(filename):
    with open(filename, encoding='utf-8') as file:
        return file.readlines()

if __name__ == '__main__':
    main()

Run daily with systemd:

# /home/user/.config/systemd/user/append-at.service
[Unit]
Description=Run append-at script (add text to files on date)
Wants=network-online.target
After=network-online.target

[Service]
Type=oneshot
ExecStart=/home/user/bin/append-at.py /home/user/py/remind/append-at.conf

[Install]
WantedBy=multi-user.target

# /home/user/.config/systemd/user/append-at.timer
[Unit] 
Description=when to run append-at.py

[Timer]
Unit=append-at.service
OnCalendar=daily
Persistent=True

[Install]
WantedBy=basic.target