OK I gave it try and it does work but is not well explained / documented:
refer
http://www.ipcops.com/phpbb3/viewtopic.php?f=16&t=9694
specifically the post starting:
In the past I asked Marco how to do it and I received this email from him. Hope this might help you too
So, to work then:
Delete all your time constraint rules via the GUI
and perform a "save & restart" from the main URLfilter GUI, then
from the post
Reset *all* settings to default -> unchecked and empty
Advanced settings -> Block all URLs not explicitly allowed: [v]
This turns off the light for everyone ;-)
Perform a "save & restart" from the main URLfilter GUI
I tested this and he's right - everybody blocked.
Now create and play with a single time constraint IP rule by adjusting the time parameters as follows (192.168.11.213 was my test PC)
from the post
within MTWTFSS 00:00-24:00 192.168.11.213 any allow
always return to the main URLFilter GUI and
perform a "save & restart" before testing rule changes.
To summarise, block everybody and then allow. Permitting all and then blocking does not seem to work as you discovered.
Hope this helps.