Zend certified PHP/Magento developer

Where to find documentation of Grub modules, such as `datehook`?

In this answer I found an example of using the datehook Grub module. But I could neither find documentation of it nor other Grub modules. Where can I find it?

(I’d like to work around the limitations of saving the default menu option with LVM/RAID: I’d like to set up Grub to “if time <= preset time then boot option 1, else option 2”. Setting the preset time to now + 1 minute or so would allow to change the boot option just for a single reboot.)