Tax years start on dates that seem chosen at random, and different countries choose differently. The explanations are historical, and they persist because changing a tax year is unusually disruptive.
The tax year predates modern accounting
Early taxation was assessed against harvests and rents, which arrived on seasonal schedules. An annual assessment naturally aligned to the agricultural cycle rather than to January.
Quarter days used for settling rents and accounts became the anchors for these assessments, and several survive in tax calendars long after the underlying practice disappeared.
Once a date is embedded in law and administration, it acquires inertia that has little to do with whether it still makes sense.
Calendar reform left visible traces
The shift between calendar systems removed days from the year in the countries that adopted it, which created a problem for anyone owing a full year of rent or tax.
The response in some jurisdictions was to shift the start of the fiscal year by the missing days so that nobody was charged for a period that did not occur.
That adjustment explains start dates that fall in the middle of a month for no apparent reason, which is a historical artefact rather than a design choice.
Administrative load favours spreading the work
Tax authorities process filings in concentrated periods, and a deadline shared with company reporting and audit seasons strains the same professional capacity.
Choosing a year end away from the calendar year spreads that load, which matters more for administration than the elegance of matching January to January.
Governments also align the fiscal year with the budget cycle so that announced measures can take effect at a clean boundary.
Businesses may choose a different year entirely
Many jurisdictions allow a company to select its own accounting period, which is usually set to fall after the seasonal peak when inventory and activity are at their lowest.
A retailer counting stock immediately before its busiest season would face a far harder and less accurate exercise than one counting afterwards.
The company still reconciles to the tax authority's calendar, so a mismatch between accounting period and tax year creates apportionment work that accountants handle routinely.
Changing the date is harder than it looks
A change would create a transitional period that is neither a full year nor a clean fraction, complicating allowances, thresholds and reliefs that are expressed annually.
Every payroll system, accounting package and reporting process would need updating simultaneously, with errors falling on taxpayers rather than on the authority.
Proposals to move to a calendar year surface periodically for exactly this reason and rarely progress, since specifics vary by jurisdiction and the transition cost is immediate while the benefit is diffuse.