At

at is a tool that schedules a job to run once, at a specific time.
You can combine with notify-send for reminders.
These jobs persist on reboot.

Install

sudo pacman -S at

sudo systemctl enable atd
sudo systemctl start atd

# ?

launchctl load -w /System/Library/LaunchDaemons/com.apple.atrun.plist
# add your username to: /var/at/at.allow

Usage

Scheduling Reminders or Tasks

echo 'notify-send "do thing"' | at now + 5 minutes
echo 'notify-send "do thing"' | at 3pm tomorrow

# NOTE: you may need to export display
echo 'export DISPLAY=:0; notify-send "do thing"' | at 4:00 PM

Managing Queued Reminders

atq       # list pending tasks
atrm 123  # delete reminder 123