Britain’s tax year begins on 6 April, which looks like an arbitrary date until you count backwards. Until 1752 the English year began on Lady Day, 25 March, and the tax year began with it. Then Britain changed calendars and dropped eleven days: the second of September was followed by the fourteenth. The Treasury had no intention of losing eleven days of revenue, so it kept its own year whole and let it run eleven days longer, to 4 April. In 1800 the old calendar would have had a leap day and the new one didn’t; the Treasury kept that day too, and the start slid to the sixth. So every British payslip still runs on a calendar that was abolished in 1752, kept going by the one office that never agreed to lose the days.