Any boundary line agreement entered into pursuant to Section 6336 or 6357, or any consent recorded pursuant to subdivision (b) of Section 6339, or any exchange of land made pursuant to Section 6307, shall be conclusively presumed to be valid unless held to be invalid in an appropriate proceeding in a court of competent jurisdiction to determine the validity of such boundary line agreement or consent or exchange commenced prior to January 1, 1977, or within one year after the execution and recording of such boundary line agreement or such consent or exchange, whichever is later.