I am not sure, but in the graphical installer, we could add a
clock widget
somewhere from the start, and do clock setting pretty early one
(we probably
only need hwclock and a little menu thingy), it can even be done
before
base-install and partman, since there are no extra dependencies.
There has been such kind of suggestion last weeks but it has been
ruled out. I guess that the rationale is mostly avoiding features that
are only available in some D-I flavours.
So, if a method to set the clock is offerred, it has to work for all
interfaces.