Update only archlinux-keyring and try again.
# pacman -S archlinux-keyring
# pacman -Syu
In some cases you may need to re-populate the keyring.
# pacman-key --init
# pacman-key --populate
Then maybe the downloaded packages are actually corrupted. You could check if they have plausible file sizes. IIRC pacman will ask you if you want to delete the non-matching files but I'm not entirely sure. They should end up in /var/cache/pacman/pkg
.
I tried syncing my times using hwclock. It didn't work. Unless maybe you can't sync your clock with hwclock. I'm pretty bad at this. I also tried the other things but it didn't work either.
You also need to sync NTP. That's the service that checks the internet to ensure the time is correct. On top of that you might want to ensure the timezone is set correctly. Also, consider reading the section on cleaning cached packages and changing your key server.