Read-only system with hwclock, but I need to change the datetime

Question

I created a read-only system from this adafruit link (https://learn.adafruit.com/read-only-raspberry-pi/ ). I have a hwclock and want to change the datetime regularly. But calling

hwclock --set --date='2018-01-31 12:00:00'

results in an error, because it has to change the content of

/etc/adjtime

Do you have any idea what I could do?

In other cases, where write access was necessary I created virtual disks from RAM. But obviously this is not a good idea since it would delete everything in /etc/.

Can I do this with a symlink to another file and use a virtual disk there? If so could someone outline the steps for me. Thank you very much.

Answer 1

I have found the answer straightforward in the documentation of the hwclock command. There ist the option --noadjfile which disables the drift compensation (it requires the --localtime or --utc option).

So calling:

hwclock --set --date='2018-01-31 12:00:00' --noadjfile --utc
hwclock -s

sets the time in a read-only file system.

I pretty sure it is also possible to use the --adjfile to give a path to a virtual disk to make use of the drift compensation, but since I update the time requlary I do not need it.