If you look at the documents at
http://www.cfmu.eurocontrol.int/ you will find this information:
It is a requirement for both ATC and ATFCM, that the EOBT of a flight shall be an accurate EOBT. This applies to all flights,whether subject to a flow management regulation or not. Any change to the EOBT of more than 15 minutes for any IFR flight within the FPM distribution area (see Annex 1 for details) shall be communicated to the IFPS.
And the reason is of course that all slots are calculated on the EOBT that you have filed. So if you donīt follow your EOBT, how shall the system know if you need to be under flowregulation or not ?