If you installed Timeshift from the AUR package (https://aur.archlinux.org/packages/timeshift/), you'll find that scheduled backups may not work. This is due to cronie being disabled by default.
Check if cronie is present first:
systemctl list-unit-files | grep cronie
This should return that the service is present, but disabled. Start it by running the following commands:
systemctl start cronie
systemctl start cronie.service
Now check if cronie is indeed enabled:
systemctl status cronie
If cronie is enabled and sheduled backups are working, make the change permanent by automatically enabling the service at startup:
systemctl enable cronie
systemctl enable cronie.service
Timeshift should now behave as intended with scheduled backups.
Thanks!