The period a tax is assessed over — which frequently is not January to December.
A tax year is the period an assessment covers. Many jurisdictions align it with the calendar year and many deliberately do not: the United Kingdom's runs to 5 April, India's to 31 March, Australia's to 30 June.
Every deadline in the system hangs off it. A filing deadline expressed as 'within six months of the year end' produces a different date in each country, and produces a different date for two companies in the same country if their accounting periods differ.
This is why a good share of the deadlines in the corpus are recorded as relative to a year end rather than as fixed dates. They genuinely are not fixed.
The tax year is fixed by statute for a jurisdiction. An accounting period is the entity's own, and a company may choose one that does not match.
See also accounting period · filing deadline · deadline
We hold this as rules of type relative-to-fiscal-year-end rather than fixed dates. See what we hold.