The calendar that lost eleven days

When Britain changed calendars in 1752, the second of September was followed by the fourteenth. Eleven days were removed from the year, and the usual story is that crowds rioted demanding them back.

There is little evidence for the riots. There is good evidence for something duller and more interesting: a great deal of administrative confusion about contracts, rents and wages. If a servant was hired for a year and the year was eleven days shorter, what was owed? If rent was due quarterly, did a quarter still mean what it had meant? Parliament had anticipated the question and written answers into the act, specifying that payments fell due on the day they would have fallen due under the old reckoning. The careful drafting is why the change worked.

The reason for the change was astronomical. The old calendar assumed a year of 365.25 days, which is about eleven minutes too long, and the error had accumulated for sixteen centuries until the date of the spring equinox had drifted by over a week. Any society that sets planting, taxation or religious festivals by a calendar eventually has to decide whether to follow the sky or the paperwork.

What the episode shows is how much of a technical reform is legal rather than technical. The arithmetic of the Gregorian correction was settled long before, and uncontroversial. The difficult part was a transition rule that let existing obligations survive a change in the thing they were measured against.

Modern equivalents are everywhere: a new postcode system, a change of currency, a renumbered set of school years. The arithmetic is never the problem. The transition rule is the problem, and it is written by lawyers.