update rule loops

Hi

I'm trying to make a field in my table (datechanged) to automatically be
updated with the value 'now()' when an update on the table occurs.

plc=# create rule datechanged_radius AS ON update to radius do update
radius set datechanged ='now()';
CREATE 22025360 1
plc=# update radius set destinationip = '212.055.059.001';
ERROR: query rewritten 10 times, may contain cycles

This means that it's going in a loop, because the rule triggers itself.

Is there another way to do this?

Thanks,
Poul L. Christiansen