Hello, I am trying to install GuixSD, and my system time is much older
than the present; so the system won't install. It tells me to check the
system clock.

Unusual distinction—I chose to encrypt the system. Libreboot is supposed
to be able to boot an encrypted system. If it makes a difference, my
config.scm has (bootloader (grub-configuration (device
"/dev/mapper/cryptsys"))). In the Guix manual, I noticed a section about
encrypted systems that looked like it was meant for after the
installation, and Libreboot has its own instructions for the initial
boot and setting up subsequent boots: decrypting to find the kernel in
/boot.
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