72 Pa. Stat. § 5341.8
Upon the date fixed for receiving the returns of the assessors, the board shall proceed to examine the returns and inquire whether the same have been made in conformity with the laws of this Commonwealth, and whether all property has been valued as provided in this act, and may revise, alter and amend the valuations by raising or lowering the valuations either in individual cases or by districts, shall rectify all errors and make valuations where they have been omitted. The board shall complete its revision of the assessments on or before the third Monday of September of each year. The assessments made by the assessors, as revised and supplemented by the board, subject to appeal therefrom as hereinafter in this act provided, shall constitute the assessed value for tax purposes of real property located in the county for the next ensuing calendar year.
72 P.S. § 5341.8