A grievance is forever barred under this subchapter unless it is filed with the Department not later than two years after the occurrence giving rise to the grievance or, in the case of a grievance with respect to the grievant’s rater or reviewer, one year after the date on which the grievant ceased to be subject to rating or review by that person, but in no case more than three years after the occurrence giving rise to the grievance. There shall be excluded from the computation of any such period any time during which, as determined by the Foreign Service Grievance Board, the grievant was unaware of the grounds for the grievance and could not have discovered such grounds through reasonable diligence.