9780899063010 ArtScroll Mishnah Series Zevachim, published by Mesorah

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Page xvi: The Second Temple stood for four hundred and twenty years until it too was destroyed, this time by the Romans.
Ezra 6.15 dates the completion of the temple to the sixth year of Darius and the temple was destroyed in the second year of Vespasian. This means that the temple stood for 585 years, not 420 years.
Page 11: Like the chatas, the asham is generally offered to atone for specific sins. It cannot be brought voluntarily.
Whether it is permitted to bring an asham voluntarily is controversy between Tannaim. Eliezer ben Hurcanus permits it and this view was followed in earlier times by Baba ben Buta (Keritot, chapter 6).