If you want to schedule a non-GUI command to run every day without using cron but instead using built-in systemd this tutorial is sufficient. However struggling with zenity to show a GUI popup the above tutorial didn't work rather this Superuser comment worked.
Long story short; for GUIs one has to run .timer & .service as user instead of root otherwise systemctl status kept showing
Failed to open display
Proper procedure follows:
cd ~/.config/systemd/user/
nano ukhan_alarm.timer
nano ukhan_alarm.service #filenames must match
systemctl --user enable ukhan_alarm.timer
systemctl --user start ukhan_alarm.timer
File contents
ukhan_alarm.timer
[Unit]
Description=UKhan Alarm
[Timer]
OnCalendar=*-*-* 19:00:00
# Everyday 5:00 PM
[Install]
WantedBy=timers.target
ukhan_alarm.service
[Unit]
Description=UKhan Alarm
[Service]
ExecStart=/bin/bash -c 'zenity --info --text="Go get tea"'
[Install]
WantedBy=multi-user.target
In case you made changes to .timer or .service file just do systemctl --user daemon-reload
Some useful debugging commands
systemctl --user status ukhan_alarm.timer #useful for any syntax related error
systemctl --user status ukhan_alarm.service #useful if bash command failed
systemctl --user list-unit-files #view all user units
Some examples to illustrate OnCalendar syntax in .timer file
## Run every day at 3:00 AM ##
OnCalendar=*-*-* 03:00:00
## Run every Monday and Friday at 10:00 AM ##
OnCalendar=Mon,Fri *-*-* 10:00:00
## Run every 30 minutes: ##
OnCalendar=*-*-* *:0/30:00