The proceedings of the state board of equalization must be presumed to be regular and the determination of such board may not be impaired, vitiated, nor set aside upon any ground not affecting substantially the reasonableness of the tax. The provisions in this title prescribing a date or period at or within which an act must be performed or a determination must be made by the state board of equalization must be deemed directory only, and no failure to perform any such act or make such determination at or within the time prescribed therefor affects the validity of such act or of any determination made by the state board of equalization unless it appears that substantial injustice has resulted therefrom.