I believe the ICAO documentation says that a FPL is only valid for the period ETOT + 30 minutes.
When you file a FPL it is checked against route closures, danger areas, military activity, conditional availabilty and so on for the period of vailidity.
If you depart outside the validity of the FPL you could find yourself arriving at an airway that is closed.
Whose responsibility does ICAO make such checking? I thought it was the responsibility of the operator to submit a valid FPL, not that of the receiving ATSU, though PANS-ATM says:
The first ATS unit receiving a flight plan, or change thereto, shall:
a) check it for compliance with the format and data conventions;
b) check it for completeness and, to the extent possible, for accuracy;
c) take action, if necessary, to make it acceptable to the air traffic services;
and
d) indicate acceptance of the flight plan or change thereto, to the originator.
While I appreciate that CFMU carries out extensive checks of validity, I'm rather dubious about the idea of your average ATSU checking "route closures, danger areas, military activity, conditional availability" for an international FPL.