Point of taxation: officer may require evidence and issue a reasoned order to fix tax timing when primary dates are missing. Where the point of taxation cannot be ascertained because invoice date, payment date, or both are unavailable, the Central Excise officer may require production of accounts, documents or other evidence, consider the effective tax rate at different times, and, after giving an opportunity of being heard, record a written order determining the point of taxation to the best of his judgment.