Every entry below is the rule as Oman publishes it. It is a reference, not a calendar: where a deadline runs from something only you know, this page says so instead of showing a date.
This is the rule as published by the registry, checked against the primary source on the date shown. It is not your filing date. Where a deadline runs from incorporation, from a fiscal year the company itself elects, or from an event only you know about, no date can be derived here at all — the figure you see is the formula, not the answer. Extensions, transitional regimes and entity-specific exemptions are not reflected. Confirm with the registry before you rely on a date.
The year is fixed by law and not chosen by the company, and that is why the duties below can be counted at all. Article 1, item 20 of the Income Tax Law defines the tax year as «A period of twelve months commencing from the first of January and ending at the end of December of any calendar year» — the calendar year, stated as a definition and not as a default. The accounting period is a separate idea in the same law: article 31 treats it as twelve months as a rule, allows a first accounting period to run for less than twelve months or for more, «to a maximum of eighteen months», and permits a taxpayer to change its accounting period with the consent of the General Secretary. So the calendar year is the statutory position, and a departure from it is not an election the company makes on its own but a change it must obtain consent for. The consequence for this page is that the two income tax duties below carry a formula counted from the end of the year rather than a printed date, because a company in its first period, or one that has obtained consent to another period, counts from its own period end. The three remaining duties do not depend on the financial year at all: the two value added tax duties run from the end of each quarterly tax period, and the withholding duty runs from the end of the calendar month in which a payment was made.