Assessment period amendment: quarterly timing and stay-period receipt rule change affect limitation computation and undisclosed income scope. Amendments change references from month to quarter in sub-sections (1) and (3), replace 'total income' with total undisclosed income in the proviso to sub-section (1), add provisos treating a thirty-day return-extension as effectively extending a twelve-month limitation by one month, and substitute sub-section (4)(i) to define the excluded period as from grant of a court stay on assessment proceedings until the jurisdictional tax authority receives the certified copy of the order vacating that stay.