[Zope] What the f... with bobobase_modification_time in a method of a ZClass ?

Hi, I want to display the date of an object built from a ZClass from a DTML method with <dtml-var bobobase_modification_time> but I got the authentification box when running this method. OK, I give my usual Manager user/password... I got an "Unauthorized: bobobase_modification_time". OK, I put a Manager proxy to this method... Always the same ! When I can view any other property as anonymous ! Note : <dtml-var bobobase_modification_time> works without any permission in any usual DTML object !!! Someone understands ? TIA --Gilles