Clearly there are no defined departure time tolerances for departing on VFR flight plans unless a portion of that flight is filed IFR. It is my understanding that departure tolerances have been devised in order to regulate the amount of IFR traffic in controlled airspace at any given time. It clearly would not be possible to predict traffic volumes if aircraft depart excessively early/late on flight plan times hence why the +/-15mins EOBT and -5+10 CTOT tolerance for 'slotted' flights exists.
It seems that since there are no rules governing VFR departures it relies on people using common sense and keeping their EOBT as accurate as possible, if you call for start/taxi an hour early and the controller does not have your details because the system only prints/displays strips for planned movements for the next 30 minutes then personally I think it would be reasonable for the controller to insist that a CHG message is sent or a refile.