Signing errors of system upgrading
Upgrading the system regularly running $ sudo pacman -Syu prevents most signing errors. If delay is unavoidable and system upgrade gets delayed for an extended period, manually sync the pacakge database and upgrade the archlinux-keyring package before system upgrade. Basically it is syncing the package database and installing archlinux-keyring package just before the system upgrade. See the following command;
# pacman -Sy archlinux-keyring && pacman -Su