Quarter calculator

Fiscal years diverge widely and the mismatch causes real confusion in reporting. The UK tax year for individuals starts on 6 April, a legacy of the calendar switch in 1752, while UK corporation tax uses a financial year beginning five days earlier on 1 April. Japan and India start in April, Australia and New Zealand in July, the US federal government in October, and most companies pick whatever suits their trading cycle: retailers avoid ending a year in the middle of Christmas, agriculture follows the harvest. "Q3" therefore means nothing until you know whose year is being counted, and the offset is the only arithmetic involved. Shift the calendar quarter by however many months the year start differs.

The quarters are not even the same size, which quietly distorts any comparison between them. Q1 is 90 days in a common year and 91 in a leap one, Q2 is always 91, and Q3 and Q4 are 92 apiece, so setting Q1 against Q4 pits 90 days against 92: a 2.2 per cent head start before anything about the business is considered. The working-day spread is wider still, and for anything driven by staffed days it matters more, since two quarters of identical length can hold three different working days between them.

Retail gets around all of it with the 4-5-4 calendar, which builds every quarter from thirteen weeks so that all four hold exactly 91 days and the same number of weekends. The cost is a 53-week year every five or six years, needed to stop the whole thing drifting away from the calendar. Where none of that is available, the honest fix is to compare daily averages instead of totals.