IPv6: Set valid_lft and preferred_lft differentily?

Installing and Using OpenWrt
1

Is it possible to differentiate between the valid_lft and preferred_lft?

I can only find this option?

ra_useleasetime:  Limit the preferred and valid lifetimes of the prefixes in the RA messages to the configured DHCP leasetime

I think here is the answer:

So the leasetime is also the preferred and valid time.

I made a first attempt to implement that: